http://www.vbulletin.com/order/
It's $160 for forever, and $30 for a year whenever you want updates. The $160 comes with the first year of updates; after that, as long as you don't want to update your forums, you never need to spend another cent on your forums (except for your hosting solution, of course).
There is also a yearly lease option of $85/year, if you don't plan on having forums for very long.