I have donated to the Walther Forum. V Bulletin costs $160 for the software and the first year of support - and then more for future years (for support and updates). It is a better software than PHP. The Walther Forum did some upgrades, and there was talk of maybe ending the forum. So, I donated.
At the Fn Forum - we take donations. There are expenses for a forum for hosting, and more bandwidth as the sight takes off. We also hope to change to V Bulletin eventually.
Now, I run the site with Therm. I am not the site owner, however. The funds go to him. However, he also pays for the expenses as well.
If people like a site, I see nothing wrong with it.
At Handgun Forum, which I used to co admin, the guy was out approx $500 before we started taking donations and was able to recoup some costs.