Hmm, I sort of get why you do it but it still seems weird that the amount of tip should factor in the original cost. It's almost like you're insinuating that people should pay what they estimate the value of something to be to themselves, rather than what it's offered at. I don't go on ebay to buy something and then send extra cash to the seller just because that thing was extra valuable to me. Like most people I look for the best price I can find and then decide if its low enough.