What would you do if the forum had a subscription fee (to post classifieds, etc)?

Of course, when the forum was small effectively the whole thing was free as it was so small it could live on a server being used for another purpose and there were no charges (for the first few months). But as the forum grew the site has required not only its own server but a second server too... so something that once was very free now has a real cost.
Then let people know. A lot of sites that require donations keep a monthly target up to let people know that this costs money.

I wouldn't have imagined this forum taking up 2 servers, my estimate of the cost would have been way off. It would also help with the animosity some people are displaying towards the idea of you making big bucks off other's helpful posts etc. as they'd see that the server is a money drain, and it's not all going in your pocket. I'm in no way saying that you "don't have the right" to make a bit of cash yourself, but the content here is user generated, the forum is just the distribution model.