I’m not sure either
(My totally blind guess would be about half of your guess, which would be plenty high enough for your point to still stand)
That said, corporate headquarters get moved frequently enough that this is probably a question that Google could and would model out were such a move actually under consideration.
I'm curious what your guess is? I'm not talking about opening a new satellite office in a cheaper area (they've already done this, for example Pittsburg) but actually moving their headquarters. Tens of thousands of roles moving from Mountain View / Sunnyvale to Kansas City.
Most of these roles are occupied by people who have a lot of options and don't want to move across the country. Even if they were up for moving and the company was offering to keep the same (far above market for KC) pay, they'd be very nervous about entering a situation where their employer would have so much more leverage over them.