A couple of thoughts about unsolicited requests to buy advertising on your site:
First, they are often an email address phishing scam. They will try to get your reply to confirm a valid email address and you'll never hear from them, other than to find out you're now on a spam list. Do a whois check on the "from" email address, and then if it looks odd, delete it and don't reply.
Second, they are also a common affiliate program marketing trick/scam effort. You reply and they send you information on their affiliate program, not paid upfront banner ads. Again, do a whois lookup first, check out the site and see if it looks like a good match.

To determine your banner ad rates, you will probably need to look at your unique page view stats, perhaps calculate your estimated number of clicks on a banner by using current click through numbers to your affiliate links you have now. If you get 500 clicks on a banner during a month, then I'd say 10 cents per click is not out of whack with current rates. So that would be 50 bucks a month for a fixed banner placement.

If you don't have that information, you might try to generate it by putting a banner somewhere you think people would pay to be, and then track the clicks on that banner. Just remember that prime placement will be the request of anyone paying for fixed banner position advertising.

If you're making more from affiliate sales than you might from fixed banner ad sales, then stick with affiliate links and banners. You definitely don't want to accept advertising from a site/sponsor that is not compatible with your site, so be choosy and be cautious. Money up front from first time advertisers is a good rule to follow.