Link Prices vary from person to person. From what I've seen, about $1-2 per 100 posts Also, depends on how many posts per day. Someone may have 300 posts but only make 1 per day, so at the end of the month they'll have 330; however, someone else may have 200 posts but make 5 posts per day, and at the end of the month they'll have 350. So you can't just judge by how many posts they've got
If you are looking to boost your webmaster forum than i would suggest you to buy sign link of some reputed member or member with good number of posts (>1000) The price for 1000+ posts sign link will cost you between $12-$15/Mo