Algorithm predicting how likely and how much people will donate?

I am trying to develop a formula for an alumni organization using a database of alumni donation history. I want to develop some kind of formula indicating how likely and how much an alumnus will donate-- a "givability score." The formula would take into account how much someone has donated in the past, how often they have donated, how wealthy they are, etc. Any ideas on how I could go about developing this formula? Thanks!