True, but... they could always tag and trade him at the end of next season. It's a high price but it's what they did with Cassel. (KC had no intention of keeping him on the tag, it was just so that his rights could be retained for the trade.) I don't think they'll settle for whatever the highest offer is; I think they know what they want, and won't take less.
Despite Brady's longevity, they still put a really high value on Jimmy, and since they drafted him in the second and developed him, I would think they want to see a return on their investment. If he walks in FA there's a chance they would get a third-round comp pick for him. (I think that formula is based on what the departing player signs for a contract, and Jimmy G will get a good deal somewhere.) They also REALLY like Jacoby Brissett.
If it is a second round pick as the centerpiece, they'll ask for more (package of the second plus multiple lower-round picks) and some team will pony up. Thinking about it, BB may ever prefer it that way.
(Good to see you by the way Jvan, hope you and yours are doing great!)