Normally, you should pay 5 bucks/month for VBulletin. At least, I've been told that a billion times.;)
Whoever has told you is misinformed. Take a look at vBulletin's pricing here:
https://www.vbulletin.com/order/
Basically, it's $85 for a one year subscription, and $160 for full, unlimited ownership. Henry over at Zelda Dimension just payed another $85 to renew his lease for one more year, according to that site. I honestly think that wasn't a good move, because for double that price you get to keep it forever.
The $85 dollar one year lease is only good to buy if you want a trial run of the software. Otherwise, the owner's license is a hell of a lot better deal.