26M, the transition tag, and let him see if he's worth more than that and get 2 1sts. If not, let him go for the gold with a new HC and offense. The circumstances dictate not getting into 100M+ guaranteed contract until they see him with this HC and offense.
He's in that Garoppolo and Cousins range, 27.5, 28M.
Someone started a thread about him signing an exclusive tag at 33M but not the transition one and I disagree, he will sign and show for the 26M. If he holds out, he will lose out and his agent knows it. If no other team is willing to bump the transition tag and give up 2 1sts for him and he is a no show for the Cowboys, he's not going to get fan support and not likely, teammate support either.
The tricky part is being able to use the safety net of rescinding the tag and bringing in a vet on a 2 year deal. I would sign Rivers and move on from him and go for the next franchise QB. However, they would have to move fast because other teams would be after him. He can still play and is as accurate a QB as there is in the league.