If the goal is to increase Google PageRank, one problem is- Google will not "credit" you with the link for 2 to 3 months! That means if you are paying say $75.00 per month, you will have to renew three or more times just to receive any
PR boost from the link.
Plus... it is rumored that Google may be penalizing sites for "unrelated" links! I'm not sure this is true, but some may be unwilling to take the risk.
On the other hand, if the primary reason you buy a link is to increase traffic, well then as long as the referring site is busy and related to the target site, you can receive large increases in traffic.