I've found that price doesn't vary too much. 10 ml will run about $6 US. If its much more than that, I just pick a different dealer.
At this point, I'm happy to have just found a guy locally who deals exclusively in vapors. Most of the vendors around here are tobacco shops who also deal vapors. This guy makes vapors for $6/10ml, and if I find something online that sounds interesting, he will just mix it up for me. Similar to my home-brew guy. I can bring him a beer and he'll give me a recipe that almost always comes out pretty close.