For 121k users at the moment (according to the website statistics posted here) it will cost €67/m, though I think we can all agree the system is a bit under provisioned at present, despite using the largest instance provided by the VPS (edit: not entirely true, see @ajar7 's reply below).
Assuming the truth is somewhere in between (I'm assuming the current state is ~30% under provisioned), and horizontal scaling will be both necessary and somewhat less efficient (I'm assuming ~40% less efficient than vertical scaling)
It comes to something like €0.001 per month per user.
At Wikipedia they get around 40,000 gifts of on average $15. They get something like 5.1B unique visits per month. That's roughly $0.00012 per visit.
The comparison seems to imply we need to get something like 10x the user/donor engagement of Wikipedia, which is very sobering.
Kata1yst