i would never pay more than $2 for a 20mg XR. Where i live, it's usually $1 per 10mgs XR. As for the IRs, usually double that, so 10mg would be two bucks.
30's commonly go for 5 on some college campuses. I think those are the people that don't normally buy drugs, and don't know they are getting their head busted.
I can usually get the 10s for about 2 bucks a piece. But most of the time I just trade some mj for them.