Most posters at conventions are priced around: 11x17: $10 - 15, it drags customers in when you make deals for them for example buy 2 get 1 free etc. 13x19: Haven't actually seen this size yet since its hard to display. 18x24: $30 - 40 mostly because its pricy to print this size in general.
I have a few 11x17 posters from SDCC, and that's a pretty nice size on my wall. I don't like the skinny posters that booths like Nickelodeon have given out in recent years (9x17" I think). I like 9x12s for just the artwork itself, not so much for putting up on the wall.