Well, it all depends on the traffic the site gets.
If you don't know this information yourself, ask the people who own/run it. I'd even go so far as to say that what they are offering is
much lower than what it should be but dependant on the traffic it generates you would like to consider the offer very carefully.
Clearly, you're currently in two minds about doing this, so any information they can provide you is in their best interests, not yours, as you'll only be making a pittance out of it.
Mark