1. Evaluate available advertising options (cpm, affiliate, # of ads per page, pages per site, etc.)
2. Create a spreadsheet with your assumptions for each of the variables (some of the things listed above)
3. Plug in different traffic volumes and see how revenue changes based on the selected variables.

Reality is typically different than whatever model you create, so another concept is to stop guessing and start testing!

My $.02.

Generally speaking, the best way to work this out is just to test it. You'll also be able to source industry numbers fairly easily I would of thought but you can't beat real data for your particular site and audience. If you're yet to launch then obviously industry benchmarks will be more useful - but you should still be able to do some testing. Another thing you can try is call a media agency that buys and sells advertising - ask for their rate card and assume 50% of what they offer as a benchmark. I'd also make sure you forecast and test lots of different types of advertising - cost per click, cost per mille and cost per action.

I've always used the metric that 100,000 people will generate $5-6k per month. Revenue opportunities and pricing will increase above the level.