I work in the veterinary industry. I can see it from both sides.
The costs of overheads, consumables, staff costs and now employer national insurance has all gone up, the expectations of owners has also gone up (people upthread have mentioned more advanced imaging, surgery, emergency care and overnight hospitalisation, this all costs).
a hospital might invest hundreds of thousands on a cataract surgery machine or over a million on an mri scanner, they need to make back money on this investment. Even standard ultrasound scanners and dental equipment needed in small GP surgeries costs tens of thousands. Wages for vets and nurses has stagnated and in some groups there have been redundancies. The vet you see is salaried, not on commission and is often being pressured to fit more consults and procedures into a day, doing admin in their own time.
However, the profits/pay scales of those up the corporate, many layers up from the vets/nurses on the ground, is less clear and the CMA is looking into it.
Also I agree with PP who highlighted the cost of some meds such as the chloramphenicol eye drops with are ~£40 in practice and around £6 otc, though it is a criminal offence now for vets to tell you to buy human products for animals without a valid prescription. There definitely are costs and wastages in keeping a fully stocked pharmacy ready for any medicine which may be needed promptly, and the time and expertise in prescribing the correct medicine/dosage. But there are some markups which still don’t make sense even taking that all into account. You are ALWAYS within your rights to ask for a written prescription.
Sadly in OPs case, it sounds like you have taken on an elderly cat with pre-existing diseases (dental and thyroid) and sadly there is no easy way of finding funding for this. I think you do need to think long and hard about what you are taking on and the costs involved. He may be better to be rehomed via a charity who may be able to get these things stabilised prior to rehoming.