During the whole development process, this was our only 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 game engine from Valve, we re-used their assets, and we also used a few (three) licensed tracks. For as long as we don't earn money out of this, we shouldn't get too much in trouble for anything. Plus, that was simply not the point of the whole experience in the first place.
Then, when everyone asked us to put a donation link, so we changed our mind, but we still decided to wait until the game was fully finished and released.
We didn't want to take anyone's money before the game's release. It wouldn't have been fair, what if people disliked it in the first place?
You donate as much as you want.
1 €, 2 €, 5 €, 10 €, that's up to you!