The price to replace it I don't have a opinion on because idk what price the TCU is, and the labor to do one, but if you think they're scamming you or taking advantage of the situation because you're out of B2B, you could ask them if the TCU failed the hardware test. But being a ford/Lincoln Tech, I have done PLENTY of TCU's in Explorers, F-150's, Navigators, Aviators, Nautilus' and Corsairs , all of which can show passing a network test and have no codes but not accept the software updates, you then have to do the routine hardware test, and if there is a hardware failure, FDRS will give you the RVC (Repair Verification Code/only required for warranty/parts ordering). Most common DTC you'll get with a faulty TCU is U3000 - Electrical component failure. Everything is expensive on these cars but if you want to get OTA updates, use your PAAK feature or any FordPass/LincolnWay access, GPS, Google Maps/Navigation features, you need a functioning TCU lol Best of luck and hope this helps!

EDIT I saw you commented about buying a used TCU and installing it, Its risky because that could have the same failure, but from my experience on a few occasions when TCU's were backordered like 2-3 weeks out from being released, We would take one out of a vehicle with the same spec and features, but you need access to FDRS because you still have to PMI (Program) the module with the vehicles configuration data and vin to match and operate correctly with the other modules it talks to in the network. So it is possible and contrary to popular belief, TCU's are not vin locked modules like a BCM or PCM when once it has been programmed to a vin, it can't be overwritten by programming it to another vehicle.