You probably wont need a new caliper. Without the pad on there, it is possible for the piston in the caliper to come all the way out which is bad. But that probably didn't happen if you can still drive it. If it came too far out, it would be leaking brake fluid bad and you would have probably lost all brakes within about 10 pushes on the brake pedal.
A rotor should run between 18-28 dollars per rotor at Autozone and a set of pads is about 19-28 for a set (both sides). The difference in price is the cheapies and the good ones.
If you take it to a shop it will probably be about 1 hour of labor, average labor is about 75 bucks per hour. You should get 2 rotors for your back to keep it even.