Let's say I have a poker rakeback site, and I want to build traffic. What would be a fair price for links to my site? What sort of things do I need to consider aside from (PR, traffic, content, target audience of his site, target audience of mine)?
find some poker blogs and ask about a link in the blogroll, throw some money at the owner and see if he takes the offer.
You have to be aware that a lot of people do not accept gambling sites. I would focus on the links that will bring you the most traffic. Check out their traffic and buy links only on sites that are related to gambling or has a potenial to attract people who like to gamble. Price is hard to say, guess it depends on how much value the link offers to you and the possible return on investment.