Construct Extra WC - Allowed? Budget?

Yes, you can add a WC
No, you don't need permission, but a quick phone call to the gemeente is prudent.
Cost depends entirely on your house and the randomness of being able to actually find a plumber prepared to do the work. Don't expect miracles.

Getting water to it should be fairly straightforward. But does the location have easy access to a soil pipe? That could be a very high cost. If it is above or below an existing WC then you are ok.