You dont have to remove that nut, the rotors are free floating on the axle shaft. they are probably corroaded to the hub. get some PB blaster and hit the area of the hub to rotor. Then take a rubber hammer or block of wood and tap the rotor to break it loose from the hub . I assume you have remove the caliper from the hub already...The rotors are reasonable from such places as TireRack.com The Brembo Rotors are of very good quality, but check around. I cant remember, but there may be a threaded hole on the rotor hub. you can screw a screw into that hole and possibly break the rotor free...
the exact hex cap screw size that fits those holes: M8-1.25 X 20 works perfectly and using 2 at the same time breaks the corroded rotor free effortlessly.