It depends on what you want. If you want one that is in excellent shape with no parts missing, than probably around a $1K. If you don't mine some imperfections, you can possibly find one from $400-$500. The type also makes a difference. Pre/early war brass ones run more than the later M1915 ones. The one I have is a M1915 version with steel spike and eagle. I got it for $395, which I didn't think was too bad. It was missing the chinstrap and kokarden, plus the liner wasn't in the greatest shape. It also has some surface imperfections on the leather, but I didn't mind any of this.
I do have one that I made from parts. I acquired a repro leather shell for around $50 and also a genuine brass spike and eagle for around $140. It doesn't look too bad. I'll try to post some pics in the next couple of days. If you decide get a repro, I'd suggest maybe doing this. You can get repros that are made in India for around $100, but they are very disappointing. The eagle and spike are all wrong and the overall shape of the helmet is incorrect as well.