I did that until they actively removed member perks. Which ticked me off. So now I don't.
It costs very little to actually host a website like this. Your costs involve hosting and software. The more traffic you receive, the higher your hosting costs, but much of the cost people complain about are salaries for people who are doing work behind the scenes. I do volunteer admin work on another site and we solicit funds yearly to cover costs for an all-volunteer staffed site. It's four figures.
A lot of forums are run by volunteers, so salary cost is a non-factor. There are sometimes costs associated with bringing in experts to troubleshoot problems, etc., and those are temporary costs.
But let's not forget that content is what brings people to a site to begin with. We are the content. We're already contributing something of value, without which the site would have no reason to exist.
Yes, you can certainly choose to contribute more, and many do. I have, in the past. But no one should feel obligated.