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.