Sure there are costs associated with running a website, a site like this should cost <$250/year in hosting costs. Software licences for web forums cost about $200 every 5 years.
I dont see problem with ads that are displayed as ads. Once you start injecting and intercepting content, a sites integrity becomes challenged.
For example, did this link get automatically inserted or did I put it there?
That's a bandaid. The real issue here is one of community vs commerce. I'm all for a community site tossing up a few sponsor ads to pay the bills -- community forums cost 20-30/month in hosting fees + a few hours a week to administer.
Once you start monetizing user content, you cross the like from community to commerce. That's OK, but there's also a risks once a site owner heads down that track.
LOL... I see you have have never run or worked with a large user base forum. A large amount of users on at the same time uses too much processor resources so you cant use a shared hosting server. A dedicated server is a lot more than $20-30 a month.