It depends on your: 1. keywords 2. niche 3. origin of the traffic and geographic location of your visitors.
If you sell slush machines on a well organized site, and you get 1000 users a day from google searching for "buy slush machine(s)", I would say you could make alot from the site! But if it's a forum, you probally would make something like $5-20 a day..
Topics of the sites: Gambling - (traffic mainly from the UK) Sex and Relationship advice Environment/Going green Dieting/weight loss Student discussion forum (also mainly UK traffic)
Gambling -> High Sex & Relationships -> average Environment -> low to average Dieting -> High Students -> Low Very generic on what you'd make and there could be wide swings depending upon the source of traffic, specific keywords targetted, etc.
You could also try posting affiliate links from CJ or Linkshare as well. These could have the potential to bring in more money....