How much would it cost today to build a full medieval style castle?

Depends on where, and what country you want to build it in.

Ask the French, since they’ve been doing that at Guédelon .

(It would be much cheaper to find an existing one.)

West Edmonton mall cost $500,000,000 to build and it is the closest I can descibe as a castle in modern days.

@RedDeerGuy1 In what way is a giant shopping mall containing a water park and a pirate ship replica similar to a medieval castle?

@Zaku It is a HUGE building. Best modern design of a Grand Bazaar. Best that I have seen in person. Sorry I never seen a real castle in real life. The costs should be similar.

A metric assload.

But no need, there are places looking to give them away, hoping people will restore them.