Well it costs me a lot less to pay health insurance on my dogs than it does for my own BUPA subscription. The cost for a consultation is comparable to a visit to the dentist, and £120 for the snip is far cheaper than it would cost to be seen privately for a human!
A veterinary practice have large overheads, staff wages, rent, equipment, staff training and wages, together with insurance.
The PDSA are a charitable organisation, and their charges are subsidised, so it isnt that the vet quadruples the fee.