I think those prices are pretty good! I usually go for smaller prints because I don't want to overwhelm my room and I like to try to shove everything on these two bulletin boards I have, but I think 13 by 19 will be the most popular size (:
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.