The sad truth of the matter is that "GVP Simms" and "cheap" are an oxymoron. GVP ram prices have held over the years, unlike nearly all other types. :-(
My advice is to check the shops, Amibench, Ebay, etc. and take the best price. Try to get 16Mb chips to buy bulk. You might have slightly better luck with the American stores, as GVP are an American company, I think. But then, you'll need to pay shipping...
