Determining what branch can be impossible unless there is a unit crest painted on them, The Army and Marines used the same helmet through the war and kept using them until WW2. There should be a stamp in the metal on the bottom of the rim that can id the manufacturer and if whether it's British or American or from somewhere else. Also check the liner for any markings. As AU Seeker said, there are a TON of fakes out there, many of them done so well that they used old paint even. It can be VERY hard to tell if the paint is original or not. If you can, please post some larger photos and close ups up the liner, the rim and the top. There are certain things to look for as far as wear patterns and patina that can help tell if you've got the real deal of if someone added the camo later.
EDIT: You asked about value, if the camo is original to the helmet, without a unit marking (which can tie them to a particular place or battle sometimes) you could be looking at a couple hundred dollars value, that's a retail price estimate though, and it may not reach that much. If the camo isn't original, it's worth about what you paid for it.