Disclaimer: Not all who make a link-buying query will actually buy, no matter how you spin it.
It varies. A lot of it depends on how knowledgeable (and how wealthy) the customer is! I've sold permalinks for $5 at PayPerPost. I've also RENTED links out for about $42/month!
Knowing the mindset of the buyer is important for this. Is it someone who doesn't care what Google thinks about link buying? Then they're not going to pay a lot. They'll probably just go to PayPerPost and pay a couple of bucks each.
But if it's some whitehat who thinks buying a link is a bad, bad thing, that's where you rake it in. Those people have the mindset that they'd have to pay a lot of money in order to "get away" with THAT. They're looking for sites that give no clue that links are for sale there. To them it's almost like paying the mafia for a fix. Plus knowing whitehats, they probably even feel guilty. So you can charge a lot more and basically milk it.
Also, if it's someone with access to the budget of a large corporation, they don't blink when you tell them it'll be $500/quarter for 4 links on 2 sites that get no traffic. But even these don't last forever. My link-tenant eventually found PayPerPost, decided that was fine after all, and my great deal went *poof* without so much as a goodbye. They just stopped renewing.
What I did for the $42/mo was go to Text Link Brokers and look up what it'd cost to rent a link on a page with the same PageRank as mine was. Then I knocked off a few bucks as a "volume discount." That worked. I figured the buyer'd certainly seen TLB (Text Link Brokers) and think he was getting a deal, whereas I was still making more than I would have if I'd had to give TLB a cut.
Read up on the whitehat SEO boards and see what line of Cutts' Crap they're believing at the time. If your blog would be a desirable location according to that, charge accordingly. When I charged the $42, home page links on old content sites with decent PageRank were all the rage. Never mind if the content site hadn't been updated in 5 years and got all of 10 hits/mo. Old + content + PageRank was all they cared about.
Pay attention to your PageRank when selling links. Often, prices are based on that. If he wants new posts made, then go by your front page's PR. If he wants to get onto old posts, look at the PR of those pages. Charge more to get onto old posts that Google already knows about.
Again, your mileage may vary.