I have a spreadsheet and small program that I wrote that calculates prize distributions given the initial prize fund, any side prizes (high game awards,...), and the number of teams. It is tunable to balance the distribution or provide emphasis to the higher-placed teams.
The spreadsheet is in OpenOffice.org format, though another user here converted it to Excel. The program, which does essentially the same thing, but is slightly more flexible, was written on a Mac. If you have one, I can send you the executable. If you're on a Windows or Linux machine, you'll have to find some kind soul to compile it for you.
SH