It's probably not a scam and is pretty common now days for marketers to buy links on other web sites.
However, do be careful. A few paid links on your site is probably safe. If you go overboard with the paid links Google may get mad at you and penalize your site. Paid link value is based on things like, your traffic, page rank, keyword value, industry niche that you blog about.
Ask them what anchor text they want in the link and this will tell you what key word they are going after. Use Google keyword tool to estimate how valuable that keyword is and base your price on that.
Hope this helps
Allen - GroceryWiz.com