It depends on the quality of your traffic, is it mainly search engine type in traffic, or is it traffic exchange style traffic. I would recommend atleast $100 a week for really targeted traffic, or $100 a month just for general ad space like a top banner, or text link etc.
I think it will depend on GEO location of traffic. I would say start slow and charge high when your traffic increases
$25-$150 is actually a great idea, it would be suitable for any type of company, so your chances are actually very bright with that amount.
If you're still looking for an answer, I would say that $30 a month would be already lucky for your site topic and current traffic.