Hey OP,
I am a vet. No time to read full thread, but my thoughts are this:
You cannot compare the costs to that of a euthanasia. The anaesthetic drug used for euthanasias is an old drug that is not as safe for anaesthetics as the modern drugs we use now, but obviously increased risk of death is not an issue when the drug is being used for euthanasia.
So for an anaesthetic for a surgical operation, a more expensive drug will be used to induce anaesthesia (in cats, this will often be a combination of 3 separate drugs combined in one syringe), oxygen and anaesthetic gas will be used to maintain anaesthesia, other drugs may be used to pre-medicate the animal, and also at the end of the op to reverse the anaesthesia. A nurse will be constantly monitoring the animal during the anaesthetic, and also during recovery. Hopefully that explains why the two uses of an anaesthetic drug are not remotely comparable.
In addition, you are paying for the vet's time to complete the operation, consumables used such as suture material, scrub solution for site of wound and surgeon's hands, bandaging, etc. Your cat may have needed to be put on a drip during the op. You are also paying for costs to maintain and sterilise surgical instruments. I could go on.....
Comparing to neutering is also inaccurate - many vets perform these at a loss, as it is a good way to encourage people to register their pets with the practice.
Having said that, over £600 for a simple wound stitch-up sounds at the higher end of things. It depends on the size of the wound, the time taken to stitch, etc, but from your description, it may well have been cheaper elsewhere. A word of caution though - best not to use the cheapest of the cheap with vets, as corners are more likely to be cut. This advice does not apply to low cost or free treatment from the RSPCA or PDSA as they are obviously subsidised, and in my experience, do excellent work.
Final word: Definitely consider pet insurance!! 