I don't think you just assume that you can't do better than Josh Thole as a backup C. This offseason, guys like Hundley, Castro, and Suzuki will all sign for <5M to backup, and are objectively better than Thole. All project at 0.5-0.7 runs above replacement over 200PA. Thole projects at around 0.3 to 0.5 runs below replacement.
That's like a 1-win swing for less than a 4M investment, and a lot more than that if Martin goes down for an extended period.