You should probably ask them what they expect you to do.
Pricing too varies widely -- by orders of magnitude -- depending on the market and your demographics.
Here's the question: Is the money you'll make worth the penalty in the annoyance and appearance of advertisement? There's always a line over which the answer is "yes." Decide that first (i.e. monthly revenue) so you can objectively evaluate the offer.
Do you really have the traffic volume to support being in an ad network? If so, you should do some research and make your own decision vs. having a "marketing company" come to you with potential snake oil claims.