Go here: http://www.sig-link.com/ That is a signature link auction site. There is even a tool to estimate the value of your signature.
Different forums may have restrictions regarding what kind of links are allowed to put on your signature, but other than that, it can be done. Anyway, it seems that people are charging anything from $1 to $10 or more per month/year... I suppose "post count" is one of the most important factors and then of course what kind of forum it is....
Yes, you may want to check out the following post for more details: http://forums.digitalpoint.com/showthread.php?t=214542 The king has offered a slot for $150 per month...
Yes it is sir, especially as soon as you can get some meaningful posts submitted and you have a buyer that is interested. The key to the game is getting good quality posts in and you should have no problem. Good Luck and welcome to DP.
That tool is showing some weird numbers. I just tried some numbers to see how mush is the worth for signatures so i typed total posts 5000 and posts per day as 15 it is showing $198. If we sell the signature link for that much all the buyers will run away .