I think XenForo is somewhat reasonably priced. I do remember when it used to only cost $140 in the past and was actually surprised to see they raised it to $160 when I purchased this license.
For how simple it is to use the software and how much works get put into creating/updating it, $160 is a good price to have access to the software forever (granted you must renew to receive new updates and support)
As for vBulletin, I would not use that software for a forum like this. I consider that forum software to be targeted more towards businesses