You can't do everything yourself. It's not like JC
buy crude oil, put it
through their refinery, fraction out the propene, break it into propylene oxide, hydrolyze it into propylene glycol and then purify it.
You could do what freesmoke do, though, and get it made up by a pharmacist. A lot of them won't want to touch it. They seem to be pretty cottoned on to the risks involved in distributing nicotine in small containers, and don't want to be responsible for the death of a small child. But I'm sure if you ask enough of them you could find one interested.
It would cost pennies to make, could be sold at a premium, and testing every batch may even be economically viable. You just need to test by the gallon PG and other ingredients separately rather than have 6 different strength x 20 = 120 different strength and flavour combinations.
Edit: had another thought. Another approach would be to just get one flavour at one concentration in bulk (whatever's most popular) and test that. If you get twenty litres of it at a time, mix it up in a large foodsafe tapped container, you only need to do one test. I don't know how much the testing costs, but you could explain why this particular liquid costs more than your others, and why it only comes in one flavour and strength.