Whitehouse Custom Color is what my friends use. http://www.whcc.com/
They may be higher on price but I doubt any of the cheap places can come close to their quality. So for really special shoots and for paid prints to sell, I'd go with them. You'll be building the price of the print into the sale so I wouldn't skimp on that one!
This is from their price guide. And they have LOTS of different sizes to choose from, and types of finish. But a 4x6 lustre/glossy is .70; a 5x7 is 1.10, an 8x10 is 2.10; an 8x12 is 3.60. So this is much more than the snapfish and the like, but the quality is far superior.