To judge the worth of the ad...
What is your conversion rate?
How much do you make per qualified visitor?
How much do you make from visitors who aren't qualified (since you said some of these are unrelated)?
How many clicks did these links get with past advertisers?
How many clicks will you need for the targeted ads to pay for themselves?
How many clicks will you need for the untargeted ads to pay for themselves?
If you don't care and are only looking for link popularity...
How are the other sites listed there ranking?
What is their re-purchase rate on the link spots?
How well are the selling sites indexed in the engines?
How do the SELLING sites rank?