you raise alot of issues. i'll just list some tips based on my experience:
1) toolbar pagerank is often in accurate for websites that sell links.
2) google places different value on links embedded in content vs top nav links vs footer links. footer links are generally worth less than most other links.
3) a link is worth as much money as it can increase your profits. for example if you are selling dirt & you get a relevant pr8 link, that is not going to be as valuable if you are selling diamonds and getting a relevant pr7 link. you just cant make the same profit on dirt as you can selling diamonds.
4) if you do link buys then try to randomize as much as possible. randomize the anchor text, page placement and urls.
5) the best link buys i have ever done are the ones that generate referral traffic.
google is not stupid and they have years of experience dealing with this type of manipulation. be careful and dont make it too obvious and you can do very well with link buys & exchanges.