Depending on how badly scuffed they are and if there's significant curb rash, you could expect to pay between £40 - £60 per wheel.
The material cost of doing them yourself will be about £30 - £40 depending on where you shop.
My brother did his own after he bought a new set of rims for his Audi A3 on the cheap and they turned out great. Once they were fitted, he refurbed his old ones and sold them for a good price.
I was tempted to do the same to my original 18" Mazda alloys, which I'd kept for summer tyres, but I prefer all weather tyres now so, which makes it all the better for me if I can bring them back to nearly new condition.
I already have a boat load of sand paper type including wet n dry, and some alcohol rub, and general alloy wheel cleaner, as well as some unused Autoglym alloy wheel seal.
I'd just need some alloy wheel/metal filler + hardener, some extra fine filler, the correct paint base code spec, primer and top coat lacquer