A single link can range from as little as $1 per year up to $500/month and beyond. There is no minimum and maximum.
Typically the more authoritative and/or the higher the PR of the site selling the link, the more the link costs. For example, you might spend $500/month for a single link from somewhere like the New York Post... $100/month maybe for a single link from a site like Bob Villa's... $5 per month for a link from Bob's Blueberry Farm blog... I think you get my drift. It varies drastically.
NOTE The above are ONLY for example purposes. Not saying any or all of those actually sell links. :)