I guess it depends on your traffic, the PRs of the category pages, and number of links on each page. I think $10-$15 a year is a fair price for a featured link. I would charge just a few dollars for regular links...this is mainly to prevent a large backlog of submissions.
Charge whatever you prefer. We charge $99 for a featured lifetime link and people pay that. It all depends on who you're gearing your site for and how unique/professional it is.
You could also take a look at the competition. Look at other PR5 paying sites... and perhaps even under cut them in order to gain more submissions perhaps. Alternatively just be competative!!!