Welcome Mike!
What you've got here is the classic
Prussian Infantry Pickelhaube for Enlisted Men Model 1915. This is the wartime version of the M1895 (see example
here) whereby all the metal fixtures were switched to grey-oxidized steel to save the much needed brass for shell production and the spike became twist-off to be easily removed in the trenches. If you were to start collecting German Imperial headgear and pickelhauben, this is where you'd start.
Your helmet however is missing some major elements, namely its
Reichs Kokarde &
Preussen Kokarde (National & Prussian Cockades) and a leather chinstrap (see complete example
here). It also appears
unissued--whereby it didn't make it to field usage--as I don't see any issue stamps (the circular mark is the maker's).
This was my first pickelhaube and I slowly completed mine. However, its visor stitch line is broken so I'm quietly looking for a nicer more pristine helmet to replace it.
As for value; it would be relative to what this helmet means to you--personally, something handed to me by my grandfather would be priceless--and to a potential buyer. To try and give you a ballpark figure nevertheless, a complete
issued helmet in similar condition--very good to excellent--could fetch up to $1,000 give or take a couple of hundreds and depending on the means--militaria dealership intermediary vs. direct sale.
Since yours is:
1/ Unissued--some people prefer the helmets that were actually used/worn/issued and have "been there" even if at the expense of condition; personally I'm OK with both
2/ Missing both cockades--up to $150 for a mint pair
3/ And a leather chinstrap with M15 grey steel fixtures--originals spares are tough to find nowadays in any condition; I remember paying close to $200 for a very worn/dry rotting one almost 5 yrs ago on eBay!
I would say
up to $600 if you were to sell it directly. But again, this is just a ballpark estimation.