Hi Sean, Before you put out any cash / trades, I would suggest that you pick up a reference book or two, or do some research online. David has a reference (see his site in the link above) and
Service Publications has Roger Lucy's
Tin Lids.
Helmets range greatly in price, with plain MK II's available at very reasonable prices, while authentic "painted" helmets being quite pricey. Be very careful, as there are many phoney painted / decaled helmets out there.