Paying for ads can pay off better if the site you're placing the ad on is highly relevant to your own. That said, it can be a double edged sword. If you place an ad on a rival site, chances are the majority of your hits will come from guests who surfed in but didn't find what they wanted on the rival site. Established members of an existing forum will rarely if ever "jump ship" to another site entirely, though they may poke around a bit.
For example, my site's a Warriors RPG forum. If I placed an ad on another Warriors RPG forum I might get a lot of hits, but if I placed one on a non-Warriors RPG forum or a general RPG forum, OR a non-RPG Warriors site, I might get even more, because I'd be offering something that their site doesn't, but still appeal to the same demographic.
For example, my site's a Warriors RPG forum. If I placed an ad on another Warriors RPG forum I might get a lot of hits, but if I placed one on a non-Warriors RPG forum or a general RPG forum, OR a non-RPG Warriors site, I might get even more, because I'd be offering something that their site doesn't, but still appeal to the same demographic.
