That is very good to hear. Is it possible to have GameCenterSolution remember customer details so they don't have to fill in information every time? (Actually I think the purchase page could be a bit friendlier/slicker in general.)
It also mostly boils down again to marketing. The Left 4 Dead sale got lots of publicity and word of mouth (I heard about it from 2 or 3 different friends) AND the game got very good reviews prior to this. A 50% price cut without the publicity and top reviews is not going to accomplish much.