I will second Apaches comments, it is not always possible to give a quote where animals are concerned, they are not a car or washing machine where parts can simply be replaced. Were the likely costs discussed with you before the treatment was carried out and was the estimate of costs initially provided for the same condition with the same level of care being provided? I can only speak for how I work, but I always obtain writted acceptance for the estimated costs before the work is done and then try to stick within the estimate where possible.
Unfortunately treating dogs with a GDV can vary from being quite straightforward where the twisted stomach resolves following stomach tubing, to massively complex and very time consuming where the dog has to be aggressively stabilised before surgery to untwist the stomach, suture it in place (gastropexy) so the torsion does not recur, resect any necrotic stomach wall +/- the spleen, deal with the consequences of a devascularised stomach wall ie septic shock and peritonitis and begin the long process of rehabilitation. Would you have been happier if the dog had died but you ended up with a lesser bill, because it is entirely possible that without the care that this could have been the outcome?
Having managed more than a few GDV cases, maintaining a good success rate comes from paying strict attention to detail. It sounds like you have recieved treatment from a vet who has given you their best attention and care, and this has allowed them to give you back a dog that almost certainly would have died without their efforts, I would suggest that you take a step back and consider what is more important, money or your dog.
Sometimes in this world you do get what you pay for! If you still feel aggreived I would suggest that you discuss the bill with the practice who looked after your dog. One other matter to consider is whether the initial quote was realistic or a guess of the top of his head? The RCVS is the regulatory body for vets in the UK but they try not to get involved with fee disputes unless the fees have been charged fraudulently.