We are starting "Link building campaign" for a website which currently has good hold on Search engines. My question is : Could you guys advise me what is a good market rate for buying links for say a site having a PG rank of 4, 5, 6 & 7. Considering we buy links on home page itself.
Q: When is a PR 5 link worth $2 per month? A: When it has one or two PR 5 backlinks.
Q: When is a PR 5 link worth $200 per month? A: When it has thousands of backlinks and heavy traffic
Q: When is PR a useful metric? A: Never.
The reasons you will see a discrepancy between the prices of a link with similar PageRank is because the Toolbar is not an accurate metric of what a link is worth.
It is not useful to create a sliding scale of prices based on PageRank. This would be similar to pricing a meal based on the calories it delivers. In the real world some 600 calorie meals will be worth $50, and another 600 calorie meal is going to be worth $5. It's silly to use calories as a metric for what a meal is worth and just as useless to use PageRank. There are many more useful metrics.
Also going prices for a site with the same pr, backlinks and traffic is going to cost different in different markets. Having high PR and traffic is not going to be the same if it is a "free stuff" site (where CPC is under $0.50) as it would if you have a Personal Injury Lawyer site (CPC is over $5).
The rates vary a lot. Best links are links which brings traffic, IMHO. The best idea is to search manualy for sites where would you like to see your link and to ask webmasters would they like to sell a link to you. Think how much would you like to pay, not how much it costs...