E-mail list advertising - what to charge?

I had an inquiry from someone who has purchased a banner ad on our site and also wants to purchase an ad to send to our e-mail list. It is for a homeschool conference he is organizing and would include a discount for our members. Since we are not currently doing newsletters, I would send this out as a single e-mail to our list. I am not sure what to charge. Any ideas?

So, I solved my own problem. I asked him if he would like to just pay for results and if he had any idea of a price or percentage he thought was fair. He offered $5 per lead that registers for the conference, which I think is really good, so we're both happy. Hopefully he will see some good results and I'll get a good amount back from it!