I want to get a link in a very prestigious geek-oriented site. My website is techie-oriented, so it'd be more on topic that most of their current links. The website has a nice PR 8, so that's good to get. But I wonder, if they really worth respecting getting traffic and some PR. The link is about $300/month.
I think the only way to really bottom-line value a paid link these days is by the amount of traffic you think it will bring. This can be very difficult unless the selling site has some click-through numbers to offer, otherwise you just have to ballpark it. If you think you can get 3,000 visitors (10 cents a click), that might not be too bad; 300 visitors at a dollar a click might be too steep a price.
As long as it was from an ontopic website I might ask if instead of a direct homepage link that they instead give you a homepage link to another page on their site. Then on that page put up a nice description of your products or services, with appropriate links to your site from there.