Its a very tricky question.
SEO is mainly death with the changes Google made which don't allow people to track results properly anymore. (the results are different for every person depending on where they are, what their interests are, past searches, etc.).
What I mean is that when I search one thing and you search exactly the same thing, we get different results.
This is why its also tricky to price a link back to a site. The reason is that nobody knows what kind of value you will get in return. If you can measure the clicks, that is another story but for SEO alone that may even hurt sites because Google is very harsh on paid linking. So I don't think many companies would be interested to be in a page or directory where you get a link back because you paid for it. Google can even hurt the site owner selling those links as well the sites linked.
I think a better approach is to sell Link Ads which is different as they only allow a fixed number of sites to be in a specific position.
Also, an ads is also a backlink technically. So instead of having more ads just rotate them more. Pricing advertising will always depend on the results the advertisers get. If he gets no results, it was just wasted money which is sadly true for most online advertising today unless it contains high quality or is huge in quantity.