Upload & Sell: Off
El-co Color Labs has always done a great job with their Internet poster special. Only catch was you have to buy two, but they don't have to be the same image, and you have to use online ordering, which is simple. Price is a couple of dollars over your goal for 20x30, but they are fast and great to work with. They also have expanded the special to include other sizes.
I did a project a couple years ago for a private school - an action shot from each varsity team for their facility, and used El-co to print it. The client was thrilled with the results and the price.