Hi Cazza so sorry to hear about your little guy. The advice you have been given is spot on, believe me Mary and Karen know their stuff!
My Tetley had a bladder stone earlier in the summer. He became unwell with no warning and within days was operated on to remove a bladder stone - the size of a large pea. He came through the op well and was up and running in no time. Make sure you get pain relief for your little guy post op and you will probably be given antibiotics, we found that Septrin was best. Despite our best efforts another stone began to form very quickly and crystals lodged in Tetleys tubes, incredibly painful for him. Please, please, please don't delay find a cavy savvy vet and get the stone removed asap.
As far as cost goes, it does differ from vet to vet but I have paid £140 for one bladder surgery and £190 for another. Post op keep your piggy warm and make sure fluids and food are being taken, you may have to syringe liquid to keep the bladder flushed through.
On the food front I have now changed my gang over to pellet only food. They will all eat P@H pellet and from time to time I mix in Oxbow Cavy Cuisine (difficult to get locally, but can buy it on line). I also only use filtered water and they get an unlimited amount of hay, plus fresh veggies, but I do try to avoid the ones that are particularly high in calcium like parsley and curly kale they just get these as a rare treat.
Good luck and keep us posted.