I’ve sent to personnel of Open Problem Garden the following proposal email:
Please add in each problem page a PayPal donation button. The money
collected for a problem should be directed to the person which first solves
One additional advantage is that this would provide a way to sort problems
by importance: most funded problem are these which seem most important.
This way your site would additionally become a central hub of math prizes.
I think, it is a very good idea.