Once implemented, it would cost almost nothing to recognize those that donate, maybe a little time manually to enter donation amounts, much of it could be automated.
For example you could have script to modify the usernames of those that donate depending upon how much they donate.
The first level could be italics, then bold, then multi-colored depending on total donation level.
Of course you would need some coding, or it could even all be done manually if desired, but public recognition is a strong motivator - it's why films have credits, people have titles, world records are kept, stats are recorded, public works recognize donors, museums and libraries are named after people, etc.
Ron Paul's Convention Speech