Ad sales come down to three main aspects: traffic, audience and your ability to sell.
Traffic: The aggregate visitors to a page / category / site and their behavior on it. Are they coming back enough to see an ad? So much that they have a risk of never leaving [the facebook social stick dilemma]?
Audience: Who is viewing the page and what are they worth? Take SPF... sure you can advertise webmaster related tools but energy drinks, computers, even dating services might also find this an appealing demographic boosting value over say a self-service web help site.
Your ability to sell: Having tons of great visitors is meaningless if you can't find your way into a company and close the pitch. Most sites fall down here -- selling isn't hard but it is a game and you have to know how to play it. Your traffic is only worth what you can gte for it.
As far as telling clients what to charge, I'd first ask if you should be the one to do this? Yes they may look at you as an expert but if you're asking here, well, enough said. With 10 years in digital marketing I still won't tell someone what to charge, rather I try and explain how I see their site and the ad market for it [like this post but much more in-depth]. I provide a few guidelines and they go from there.