You could always buy your PG and/or VG, flavors and other ingredients and tools, utensils and such from US vendors to save money. s far as I can tell from the laws no of that would be confiscated and can be sold across the US/Canadian border. Then spend the extra and by the nicotine in Canada - that will increase your per ml cost but not a as much as it would if you bought all in Canada.
While saving money is one aspect of DIY, it may not be the main reason for doing so. I know some people that spend so much on ingredients that the per ml cost would be something no one else would pay but they want and enjoy that level of ingredients. To me that is like different cuts of meats - or even different brands - most are as satisfying and taste as good as others even if one is much cheaper. Now a days I can buy e-juice in flavors I could no make any better for not much more than I can make it for, and not have to take the time to make it, deal with the mess, security and safety of ingredients and everything else. Which is why, unless I absolutely get a flavor in my head I can't find anywhere, or I just want to have fun - - I no longer do much DIY.
Quick little computing tells me that if you got everything from the US and the nicotine from Canada in most cases you would save a decent amount of money with DIY.
Even if it does not really save anything there are lots of other reasons to DIY too. One, if you do it yourself you not exactly where the ingredients came from, how they were handled, measured, mixed, and the date certain they were made on and so forth. To many people that is important and why they make hamburgers at home rather then buy them from McDonald's.
Some people, and its true for me have a lot of fun with DIY e-juice. Some like the involvement of being very hands on experimenting, playing around with flavors and such. I think its like people who like to create cook/bake or sew clothing or mod their own deices - its hands on patr that they can really delve into the steps behind the finished product.
Also, with DIY e-juice you can make and have the quantities that are right for you. If you are one of those with an all day vape you can make your adv in larger quantities than you could buy them and neither have to order so often or make so often - or the reverse if you like it fresh from a steep but not a month old you can make stagger your mixing at two week intervals in the quantity you would us in two week so that the day you create a new batch is the day you open up the one from two weeks ago.
And what happens if you do have and do an ADV and that flavor stops being made or the only place you can get it any longer does not taste like it was from other places - DIY is the answer there.
The reverse is true too, if you don't have/want an ADV and are a vaper who likes many different flavors during a day making up a large amount of your PG/VG and nicotine base to then divide into smaller amounts to flavor as you want is sometime much better then tracking 10 orders from 10 vendors for 10 different flavors and hoping all will arrive on time and in close timing to each other so your palette does not end up with missing flavors.
Of course, if you get the idea of a flavor you want and its not out there it maybe that you can satisfy that urge by making your own - or in my case often find out that my brain just thought it would be a good combo because I could not find it rather than I could not find it because someone else tried and it did not work out
It comes down to individuals again, and their reason for wanting to try or stick with DIY may differ quite a bit and some may try it and not have it work well for them while others find out that for them its the only way to do e-juice.
I would actually think in Canada that DIY (and getting cheaper prices on anything you can get in the US) would almost be a 'have to' if your an avid vapor. From what I see, you can get e-juice with nic in it (although sometimes so hard to find many think you cannot) but it costs - because it either has to be produced there or it has a high internatio cost for import license into Canada and its expensive and takes a bunch of hoops that cost money to jump through to get it there. And that is all rolled onto the consumer. Also, if buying e-liquid with nicotine in it so that the only way to make sure and not custom officials end up with it you have to really look for a vendor, don't have as many options flavor wide (both how many different ones are produced and how they differ between suppliers) that you might not be finding what you really like, are waiting longer on stock, hitting more OOS problems and so on. DYI means you avoid all that. Want a certain flavor and its OOS everywhere but the one place you already know does not make it to your tastes and frustration sets in... better to be able to go decant the need ml's of a PG(/VG) and nicotine base you have previously made, add the amount of flavor to your taste and you have minimal frustration and your e-juice in your hand without dealing with OOS, artificially inflated prices, basically being at the mercy of a more limited market then you would have south of the border.