You have every right to ask your vet for a prescription and then find a pharmacy that is willing to let you have them. My local pharmacy has no problems (in fact, is rather intrigued!) in providing Oliver's gabapentin: monthly prescription from vet £7.50 (apparently as gabapentin is not licensed for dogs, they can't/won't prescribe for more than a month at a time), 90 gabapentin, £24. I'm looking round for a chemist who can do it cheaper, but can only find £20 - the price of the 100mg has recently rocketed because there seems to be a shortage. No shortage of the larger capsules, apparently!
I know some people on this forum get gabapentin without prescription from overseas, but I haven't yet explored this avenue. But your vet is ripping you off! If he refuses to give you a prescription (and it is your right) you may have to find a different vet!
Kate, Oliver and Aled