Looks like a nice Fahnrich model Garde helmet. Very nice condition also! Now about the price and others will come in also with their opinion but I would say in the range of $1500-2000?? :-k
My opinion on prices or haube "market" value is that it doesn't matter to me. Its really about how much it is worth to you and of course can you afford it. A smart collector in almost all cases isn't going to spend a insane amount of money on a common helmet when you can just wait for another one to pop up in the next couple weeks.
The seller has probably convinced himself it is an officer's helmet (even grey M15s are officer's to sellers) so if you explain that it is not, it for a Fähnrich, perhaps he will drop the price a little. Its a beautiful example.