Hi Telmos and welcome to the world of DIY.
Ready-to-vape e-juice manufacturers charge what they do because we the consumers will pay it.
We pay it because it is convenient.
It is convenient because:
-It is ready "right now"
-The work of creating a recipe has been done for us. (All the trial and error, and product used in developing a single recipe has been done for you.)
-If you have access to a brick and mortar store; you can taste (often for free) several/many different flavors until you find one you like. (Again; without the work and expense.)
-lack of know-how to safely DIY
-lack of desire to DIY
-physical/emotional reasons to not DIY
It is not the proprietary "way" they are mixing that you pay for. It is the time, research, development (trial & error), test marketing (imagine making something that tastes good to you; and then imagine making something that will be liked by thousands), production, storage, and finally distribution. All of a sudden you can see why the costs are not just the $1.10 worth of liquid plus the price of a dropper bottle with a cool label.
As you might guess; I don't hold any ill will toward the juice manufacturers.
But, you are also right, that (because I don't have any of that overhead and I am not trying to make a living by selling it) I can, and I do, DIY for a much lower cost per milliliter (at least on the face of it).
A few, I imagine, do have laboratory facilities to actually develop and manufacture their own proprietary flavorings and/or bases (PG/VG/nicotine solutions). But this is very costly. The majority will use product already on the market that can be purchased at a discount from wholesalers.
The good news is that we too have access to the majority of the same flavorings and bases the juice manufactures do.
For the reputable manufactures, dealers, and resellers; this will be more than a claim. Any vendor that I have purchased through (particularly flavor mfgs.) either already have MSDS (Material Safety Data Sheets relating to known hazardous components, safe handling, storage, and emergency response) documents on their website or will gladly provide them when requested.
Many will go beyond and provide Certificates of Analysis (CoA ) which can offer testing for specific undesirable compounds and/or levels of purity. I see these most often with nicotine solutions.
Correct. Responsible flavoring/base manufactures, especially ones who do not shun the vaping community,
should offer a disclaimer to that effect.
At this time there are no completed studies (I don't even know if there are any studies under way) on the effects of long-term inhalation (vaping or otherwise) of the materials we now use in DIY. Long term studies will take years, if not decades, and millions of dollars (in my ignorant opinion).
With vaping in general being such a small percentage of the population, and DIY being just a fraction of that, it just has not been cost effective/"good business" for the individual companies to invest in this. The same goes for developing "vaping approved/certified" materials.
So, as adults choosing to vape, and DIYers, we try to educate ourselves, share what we learn, and try to make informed choices based on risk levels we feel comfortable with. Some feel
anything is better than smoking, others will choose to avoid additional chemicals/compounds in the different material we use to DIY, and still others do not want any of the risk and quit altogether. This is a personal (and different) choice for each and every one of us. So research, read,
and when you can't find answers; ask. Then decide what is right for you.
I will say that I found
this recent publication by the British Royal College of Physicians personally affirms the choices I am making for myself. Where, among other encouraging statements, they say "However, the hazard to health arising from long-term vapour inhalation from the e-cigarettes available today is unlikely to exceed 5% of the harm from smoking tobacco." So I feel I have made positive changes from my 30+ year, pack and a half a day habit, that I was unable to permanently shake before I began vaping.
The other thing I like about DIYing is that I DO have choice over the ingredients that go into my juice that I didn't have buying premade over-the counter juice.
As I said, no manufacture/vendor (US or otherwise) is offering testing and certification as an inhalation "approved" product. But many of the reputable ones will offer MSDS and/or CoA either on line or upon request.
So, to wrap this up, educate yourself about what is required to safely DIY (there are already lots of good threads on ECF regarding this... I don't need to make this post any longer

). And, if you decide it sounds like a good fit for you, try it.
Start small. Find two or three recipes of flavors/flavor profiles or clones of commercial e-juices that you think you will like. Buy only the equipment and supplies necessary to make those recipes (and don't neglect any safety equipment you don't already have). Choose one of the free recipe calculators that are readily available. Then try mixing a batch or two. See if you enjoy the process. Then you can start to expand your horizons. (If not; you don't have a lot invested yet.)
Take LOTS of notes. Write EVERYTHING down. Keep a RECORD of every step you take. (See where I am going with this?) The only way you will be able to repeat the good results, and avoid repeating bad results, is to be able to know what you did and how you did it.
Don't be discouraged. The first-time DIYer often doesn't turn out a perfect juice right out of the gate. But, with all your good notes

, you will be able to describe your efforts, and ask questions, and get help from the terrific community here on ECF.
Best Wishes and good luck!