Sites with their own advertising units, like Woot and Slickdeals, generally price it as a function of their audience demographics, unique visitors, and total views over the length of your ad campaign. Sometimes they'll have a standard rate card. Sometimes it's only available on request, or negotiated.

Google AdWords are pay-per-click, and are priced based on the competition for that particular search term your ad appeared for. You'd have to sign up to be an advertiser and test brand names, product names and model numbers to find out.