hotel, where you have booked room said you parking will be 900 czk/night and knew you are their guest? Parking place for guests should be usually free.

Anyway, parking in prague in center or near historical places is always expensive, and let car somewhere just on street is not good idea as for safety and also its not easy to find a place.

In prague, parking areas are divided to some zones and on every, there is some ticket machine, for example here http://www.praha1.cz/cps/doprava-a-parkovani-3509.html you can find some prices for parking in Prague 1