I'm no helmet expert, know just enough to get myself into trouble.
To me it looks legit and original. It is in very good condition. Value depends on a lot of things, is there another decal on the other side? Most M40's only have one. With the rolled edge, this was produced prior to mid 1942 unless it's a Quist, they continued till the end of the war.
Value again, I'm no expert but prices have skyrocketed on these in recent years. $500+ would be my guess.