on average, not really that much. on the order of pennies per hit. minus the overhead of server / staff / lease /insurance costs. The real money comes in when you can get people using their credit cards. ebay are masters at it since they get nickels and dimes per purchase with thousands of purchases a day.
A forum site is mostly for promotion, not for big profit....however if you can get profit from the promotion all the power to you. (the way you do this is by somehow devising a way for more people to visit your site, ie: more hits) There was considerable finacial commitment/risk involved in starting a site like this...it's been up for 5 years, I hope they are turning a decent profit by now. with the articles and services / work they provide-they deserve it.
so in short, to answer your question it comes down to hit-count. more hits means larger audience...larger audience means larger companies will advertise and therefore be willing to pay more for high profile web site ad space.
look to the right >>>>>>>>>
see the ads?
hope that helps.