"I've noticed most FTOs in the area charge around 160-190 p/h for IMC instruction these days. Can someone explain exactly why IMC instructors cost so much to carry? Do they cost more to employ?? "
The reason, I suspect, is two fold....
...It costs more to have this qualification, as you need to have the Applied Instrument Instruction restriction removed to teach IMC which is more work and money.
and
...There are fewer around so they need to be remunerated better to keep them. If PPL instructors are 'rare' these days, then it would be easier to nail jelly to the ceiling than find a decent instructor with the AII restriction removed.
At the moment, TPC at Denham don't charge additional amounts for the IMC qualification, and to cap it there is no hidden fuel surcharge either!