Two-gap linemen generally have very long careers. It's hard to compare a 30 year old 2-gap defensive lineman to a 30 year old running back (for instance). To call a 4 year contract for a guy in his prime as "insane"...well, beats me, but call it what you will. As an example guys like Sam Adams, Pat Williams and Keith Traylor are all high quality players deep into their careers. Payne works out at the facility daily so I would think that the Texans are in a pretty good position to make a decison on Payne. It's not like they are bringing him in from another team and have not had the ability to monitor his progress.
That said, the Texans are going to make an announcement of a signing in a few hours.