During the whole development process, this has been our opinion:
Our goal has never been to make money out of this work. We're not going to ask money and not even to accept it. We used a proprietary 3D engine from Valve, we used their content and we also used a few (three) licensed music. For as long as we don't earn money out of it, we shouldn't get too much in trouble for anything. Plus, that was simply not the point of the whole thing.
Then, when everyone asked us to put a donation link, we changed our mind, but still, we decided to wait until the game was fully finished. We didn't want to earn money before the game was released. It wouldn't be fair, what if people disliked it in the first place?
You donate as much as you want.
1 &euro, 2 &euro, 5 &euro, 10 &euro, that's up to you!