Not much, usually. The GI helmet was starting issue in 1942 or so, and continued to be issued til the mid-1980's. I see them at gun shows for around $20. Now, if you have one of Patton's old helmets, the price goes up. Aank! Wrong. The WW2 helmet is different from the ones used in Viet Nam. A good collector can tell the difference. The earlier helmets had two unique items. The thing band of metal around the bottom edge had a seam that met at the FRONT of the helmet. Later in WW2, this seam was changed so it was in the REAR. Earlier helmets had fixed bale that the chin strap was attached. Meaning it was welded to the helmet. Later helmets issued in WW2 had swivel bales that were loose. A nice example of a helmet with original paint are now selling for close to $100. If they have a nice liner or a painted marking, such as red cross or such, that can sale for >$150. German WW2 helmets that still have some paint and the insignia on them are in the range of $250. German paratrooper helmets go for $800 or much higher. Japanese WW2 helmets can still be bought for around $300 but their value is rising very rapidly.