If you are planning on expanding certain features for members who want to subscribe, that would not be a problem. XD Talk does this and I may go that way in a while. This may be something you want to consider down the road. Otherwise, it is up to you to decide if you want to ask for donations. I would wait awhile and see how things progress. Just MHO.
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.
Speaking from experience on the Reefing Forum that I am an admin on all funds taken in are put right back into the up keep of the forum, hosting fees, server fees and now new software license fees. And while all of us enjoy forums whether here or somewhere else there is the unseen operational cost. If a forum offers extra perks for donating I am okay with that. For the forum owners to cover these cost they can charge a membership fee, bombard users with adds, or take donations. So for me the donations are the lesser of the three funding ways.
The bottom line is simple. Webspace has costs. To run a forum needs space on the web or a server owned by the person who sets up the forum. It also takes a lot of someone's time to set one up and then administer it. There is also the annual cost for domain registration. So, it isn't free for someone to set up a forum. Of course it's fair if people who can chip in some for the operating costs do so. That helps keep the forum alive. Some forums manage to get some advertisers to chip in. Gun companies often will contribute to a gun forum as a sponsor. There will simply be a logo along one side of the page under the sponsor list and clicking on the logo will take you to that companies web site.
Forums generally don't really make any profit and if they do, it isn't much.