This relates to an economic principle called Price Elasticity. The Wikipedia article is pretty informative on the subject http://en.wikipedia.org/wiki/Price_elasticity_of_demand

What you've done is some basic "elasticity testing", which, in the "real world" can often be difficult or impractical, but lends itself particularly well to online transactions of relatively small dollar amounts. More people selling small services online should be doing this - traditional businesses would love the luxury of doing it so easily.