Hi Stephen,
I think it depends on the purpose the buyers are buying the links for.
1. They can buy it for traffic exclusively, which means they're not interested in PR passing nor SERP benefits. This would mean you could check out the biddings for keywords related to your site and then multiply what you get with the approximate # of visitors you'd send to your buyers' sites.
2. They can buy it for both traffic, PR and SERP benefits. This would cost more but IMHO they won't help much your buyers' SERPs ( MSN not included here as it is known atm as a huge sitewide link eater ).
If you sell just the traffic off your pages, make sure to use rel="nofollow" within those links. ( this way you can avoid suspicions of selling PR ).
Hope this helps a bit. Wish you luck and as much $ as possible
Best regards,
Radu