I recently had to order a variable number of photographs and get them delivered to different locations. I had to do some analysis to figure out which photo print provider works out cheapest for a certain number of prints.
The above graph summarizes my analysis. All prices are for 4" x 6" glossy/matte prints and factors in delivery charges (that is where different sites provide more/less value based on number of prints ordered). The cost does not include service tax but that should not matter as all providers charge a flat percentage of the total cost. All prices are as of 10th April, 2010.
Based on this analysis, www.zoomin.com provides best value for a low number of prints (1-10), www.picsquare.com provides best value for nearly all other ranges except for between 55-65 prints where www.snapfish.in has the edge.