Somethin like 40-80$, depends on how big it is, and how much effort is putted on the code and design (If its scratchy could be even less than 40$).
A very regular donation system costs like 10$, lets say its just html & php without even css..
But depends on seller afterall, if you get nice with them they probably go cheap with you.
It is also depends on the code quaility and how its done.