Depends on the model and what's wrong with it. To give you a guide there was a twin GA plug version with broken plugs that went on ebay for about £100 and a well worn helicopter version went for about £125.
It's for my son who is in the air cadets, he has just been awarded a place on the Pilot Scheme course at Tayside Aviation and he would like to take his own headset. He's doing A-level design technology and would love something cheap that he can repair