If you search online you find a couple which will give you some price base idea. Just make sure the one's you quote are exactly the same as yours.
Vintage fighter pilot helmet ZSH-3B Armored USSR air force mig | eBay
zsh helmet for sale | eBay
1983 ZSH-3B armored pilot helmet SU-25 Mi-24 Mi-8 Afghanistan war [RESERVE] - DEPOTMILITARY.COM