A link is only worth what you are willing to pay for it. Don't worry about market value, worry about what value the link brings to your site. Will it get you actual referral traffic or is it just going to get you some nice spamtastic link value from Google? Also, how is the link going to be included. In content, in a "sponsored links" section? Of course, there are so many quality free links to be had these days, I don't see why you would buy.
Many free quality links? I guess it depends on how you define quality If this were the case then SEO would be EASY and I think many struggle with it
It depends on return of investment of the link. I really doubt that average price even exist. Each link should be estimated separately. To get more specific answer you probably should specify type of link and site where you want your link to be placed (like forum links, article links from article directory, article link from respectable niche site, etc.)
it doesnt take a rocket scientist to figure out the purpose of an "online casino" or "buy viagra online" site, just to give you example