Torpedo7 have XT Centrelock rotors for $50.
Be aware that some of the lower end Shimano brakes use a wider track section on the rotor ( the area the brake pad contacts ) and the pads are also wider, so if you use a rotor with a narrower brake track you can get odd pad wear.
That said, I was warned against using XT Centrelock rotors with my Deore 555 hydro brakes and they are still going strong after 4 years.