I am not sure exactly what you mean about the flavor's being keepers - do you mean the ones you received and tested taste/smell bad or the ones you mixed up from them were not what you expected?
First, 'flavorings' can be hard to work with in the sense that what they taste like in undiluted form (yet added to anything) can be horrible. So for the most part what you taste in the bottle of flavor extract can be far from what it ends up being when properly diluted and part of a 'recipe thankfully given what it tasted like before that)'. This is very much why professional food people will often cook/bake with flavoring direct from the source instead of using a flavoring extract (as well as to avoid all the stuff they don't want in their foods that comes along with the flavoring). It may look like extra steps or a chef being too fancy when goes to the pricy vanilla bean for his flavor rather than measuring it out of a bottle, or using the fresh juice/peel from citrus fruit and so on.
Second, flavorings also change flavor when mixed with other flavors and other products, especially other flavors, their flavors can change -weaken, strengthen and turn depending on shelf life and preservatives used. This is why bakers, chefs, etc tend to use flavorings all from the same source if not creating their own fresh extracts. Things with a shorter shelf life so they cannot be stored in bulk, and therefore cheaper, amounts do cost more. I would worry that the flavoring - since so very little is used, as a bulk purchase like you seem to be thinking wuld end up costing you the same as the flaring would just go bad before you used it all.
Unless a flavor is out of date - if its the base flavor itself you should hold on ti it even if you find it tastes really bad in its own bottle. Like I said some flavorings until diluted or mixed with other things or you add sweeteners or acids (like citric acid) just taste gastly. But sweeten them up or dilute them in a bit of sour or mix them with a few other flavors and they can be great. You could find that that first flavoring you bought and that was terrible, mixed well with something you get later on that might not be what you expected.
You have to take a look at what type of flavorings you are buying and what is in them and understand what the ingredients means. For example, there is a wide range of difference between organic, natural (not the same as natural) and artificial flavors and depending n the type of flavoring you use they can be constituted in or produced by using solvents that you don't want to inhale. Some flavorings contain food colors, some in small enough amounts they don't even have to be listed on a label let alone what things go into making those colorings and whether they are natural colorings (say from fruits) or artificial. While the food colorings are diluted in the flavoring and then diluted when it is the amount is small enough not to do damage when ingested but the respiratory system is much different and things can get into your body through inhalation that cannot get near it via digestion. Even taken by mouth some food colorings have been found, in the past to be cancer risks and many are still being monitored by the FDA - and while considered safe in food they are not approved to be used for inhalation, not not approved but not approved either. Vaping e-juice is a new things and so not everything is known about the effects of some of the things in there although what is there is safe for human consumption and people have swallowed, chewed, rubbed into their skin and they have not caused general problems or found to be unsafe by those methods - less is known about flavorings - even those approved for medications by the FDA for oral consumption.
Besides the things we don't know about in flavorings except to know they are there are the things we know about and should question. Alcohol for example. Some have a relatively large amount - for example vanilla extract hsa enough that some people with severe alcohol problems and low on cash have been known to drink it straight. Not a good thing and it can make them pretty sick.
Yes, it is often mentioned that vodka be offered as a rinse for steps in cleaning but many people have gotten away from that. Its not that drinking alcohol cannot be inhaled (some medical procedures used to do this) and its not dangerous to use it as a method to speed up drying of wicks and atomizers (although not really necessary its just a personal choice) since by the time the atomizer and wicks have dried the alcohol content has mainly evaporated off and what little might be there still is gone with a dry burn. However having it as part of the e-juice which keeps refreshing n the wick begs the question of how much is inhaled - its 'evaporated' off the coil and becomes part of the vapor.
There are also some flavorings that contain perfume components, such as lavender, vanilla and others - since part of tasting a flavor needs 'smell' component to register on the brain as that certain flavor. Some flavors have to be produced using various solvents - sme are chemical solvents - to extract the flavor and while some of these make it into the flavorings they won't hurt (that we know of) if swallowed but no one has had a chance to study inhalations. You also have to remember that USDA or FDA food grade is much different is production guidelines from USP approvals.
Flavorings can also contain food grade preservatives, especially ones with long shelf lives and even a few I have bought that were certified organic contained a preservative. A preservative that will not harm the body when eaten may have an effect on lung tissue, a lot of this is unknown so you might not want to risk it. And because you don't neither to vendors and so you see larger prices for flavoring
So you have to be very astute about what you use when it comes to flavoring your e-juice. Unfortunately, the better organic flavorings without additives or colorings or whatever are more expensive. Thats not to say that you can't get ones as good on the cheap or that just paying a large dollar amount means they are perfect - you have to learn to research them so you know.
I have done a lot of DIY but not in an attempt to save cost. For the most part the may 'standard flavors' I can buy custom made as cheaply as I can make them unless I make them in such large qualities that they are aging on my shelf much longer than just buying them as needed, letting them steep a couple of weeks and then using the 'fresh'. I got into DIY for the 'fun' of it, the hobby of it and to see what recipes I could come up with that I liked or if I could improve on some I thought would be good with a bit more of this or a bit more of that.
I did find that I ended up buying my basic flavorings form all over the place and from various vendors depending on the flavor itself. I found some flavorings that met criteria I wanted to stick with such as not having certain additives being carried at one vendor right next to other flavors that did - so those I had to shop for elsewhere. I really like lavender, for instance, but had to spend a great deal of time finding one that had a food based 'perfume' over a cosmetic grade one. I only buy that one flavoring from that store as the others they carry contain a great deal of preservatives.
I found that flavoring was the major expenditure for me. I know some people that just have a couple of favorites and make large batches at a time can really hold their costs down, I have seen their numbers and can see its a few pennies a ml. I know that you can save some cost by making up large amounts of unflavored with a dilution that will allow the addition of flavoring without diluting too much when the flavor is finally added at a later time. I was able to get very large quantities of PG/VG at very low prices going locally for PG (hardware store) and VG (pharmacy) although there was very little difference in price for the nic base. But the cost did add up with the flavorings and as they did have nearly the shelf life that the other ingredients had and so I did end up tossing flavorings simply because they were 'out of date' long before they were used up.
There are great links to lots of vendors that people have given you, and if you have just a couple of flavors you use over and over to make your e-juice to your taste then one will have all the ones you want. If not take the time to check out all the vendors and locate the best option and flavor you want from several or many vendors til you have the basic ones to work with. You will also find that the vendors themselves can be very helpful if you work with them on the phone or email. Many flavoring labels come with sparse lists of ingredients and give no clue how they are processed while the vendor will have that information from the maker.