Pulp Guns 1 has slightly more detail on price conversions from historical US$ to GURPS$. If you run into historical prices in British £, the exchange rate during the period was pegged at US$4.85 to £1.
If you want
lots of prices for the period, a department store catalog is really useful. You can sometimes get them as PDFs on eBay, or as used books via the
http://used.addall.com/ search engine.