I still haven't found a store in Vancouver where there are olive oil and balsamic vinegar tastings, unfortunately. But...

As a rule of thumb: cheap is usually not good. A 500 ml bottle should start at $10.

We have a bottle of Colavita opened right now and it's much nicer than the previous one we had, which I didn't pay enough for.

Can't help more than that. In Montreal I used to know a lot more about them, there was a store with tastings 5 minutes walking from my place. Those days are gone...