The supply of authentic wartime German helmets is limited. Collectors know what they have and prices are strong. If you just want a prop, the Finns used M35 and M40 model helmets and a company called International Military Antiques (IMA) in the States sells them for $80 and up in various conditions.
https://www.ima-usa.com/collections/...-helmets-hats/
BTW, I have one of their repro Fallschirmjager paratrooper helmets for sale in Canada if someone is looking. My head is just a bit too big for the 7 3/8ths size.