I understand that many don't feel Onwenu is "worth it" at RT and is only worth a top contract to play RG. In a vacuum, I agree. But look at what this team needs...
1) QB
2) Starting outside WR
3) Second starting outside WR
4) LT
5) RT
6) Starting outside CB opposite Gonzalez
All of those positions, IMO, are needs heading into this offseason and those spots along with edge rusher are probably all the most important/expensive/premium positions in the game.
It's really just an awful position to be in with so many holes at so many important spots. And even if you draft someone highlight or pay a huge price for a guy, there's always the risk it doesn't work out. IMO, even if he's not an all pro at RT, re-signing Onwenu to play that spot is just the safest and simplest way to fill one of those massive roster holes.
We have tons of cap space and it isn't a great FA class anyway. I would just bite the bullet and pay him to play RT. Keep Sow at RG where he looked ok, keep Strange at LG for now, Andrews returns at C. Longer term, I'd eventually want to transition Strange to C to replace Andrews, Sow back to his college LG spot, Onwenu to RG where he can be an all pro, and a good young RT to really lock that spot down. But Rome wasn't built in a day and having a decent OL is crucial part of developing other offensive talent on the roster so just take the steps to do what you can in the short term.