Glad you've found the site useful! :)
You're right - there are two kinds of costs. The hard costs of hosting and the soft costs of time and energy.
We're in a much better position that we were a year or two ago - we're now hosted on Google AppEngine Cloud instead of dedicated hosting and the time spent on the redevelopment to enable this has reduced the hosting costs which the ads, a necessary evil, now cover. I wanted to make the forum sustainable - sometimes I have received donations to help with the site which I'm very grateful for but long term I think the best model is ad-supported. That way if we ever decide to move on it can still continue under different management. At the very least I want to finish off some of the admin tools so we can get more moderators on board to help support the community we have.
I don't think it's fair for anyone to charge for their time when it's volunteered. This isn't my full-time job and it sometimes intrudes time-wise (or I have to ignore the site because work beckons) but it's my time to chose what to do with and if I chose to spend it working on the forum then that's my choice and I don't think it's fair to expect other people to compensate me for it. I do get something out of it - I get to try out new technologies and learn from running the site which is valuable to me professionally. Right now I'm building out a new version based on a Go backend and Angular2 UI - not only should it be slicker to use, quicker and make some new features easier but it will also reduce the hosting resources further which always makes costs less of a concern (or what usually happens, it makes it easier to justify splashing out on some services / features).
At the end of the day, I'm a geek and coding is my hobby so sitting in my basement fiddling with code and trying new things out is what I do for fun. Yeah, I know that sounds sad but I like it !
People have questioned why we don't run on commercially available software and to be honest, I've looked at it and the costs would be too high. Things are getting cheaper but very often the tech is based on having a server and database hosting which all costs money. It get's harder as a site get's bigger (you can start a forum for free, but it's harder to keep running when it get's busy and larger). The custom cloud solution we have now is very efficient and allows the site to keep running.
Even the fairly regular attacks we get and DoS attempts are insignificant - there is so much caching and cloud resources they just end up testing out Google's data centers and, well, I know who's going to run that one.
So things are going pretty good - expect some new features, fixes and upgrades in 2016 :)