What should I charge somone to Advertise on my site?

Please help. I don’t know where to start. I just got an email from a major retailer near me that wants to advertise on my site. I would love to have advertisers but I have no idea how much to charge. I run a community website from our 900 home development an I can get 100-300 hits on a typical day. More when I send out an email blast.

Your suggestions appreciated!!!