My son tripped over the cable cord and the cable/antenna input came off. Is it repairable and is it costly to get it fixed

Should be some what of a cheap fix but it depends on how the RF connector is mounted. Some are just connected to the back with a cable hooked to the tuner and others to the tuner itself. If its hooked to the tuner, then the tuner has to be removed to replace the connector because the connector has to be resoldered to the circuit board and the metal shield. If its on the back then its just switched out. If the circuit board in the tuner is broken then the whole tuner will have to be replaced and that can cost alot more, if the tuner is available. Shouldnt cost over a hundred dollars to repair depending on the repair shop

It depends if the connector broke the tuner it is attached to. Most times the chassis of the set would need to go into the shop. I would generally charge $80.00-$120.00 if the tuner was still good - a new tuner would cost $60-$100 more if bad
The better solution is to use the video audio inputs (if on satellite or cable box) or use the video inputs by changing channels through the VCR (if on cable or antenna and a VCR is available)