In England the finder gets a good split, about 50%, I think.
In the U.S. treasure finders often get nothing if they report it to the authorities.
Sometime finders of fossils are prosecuted and jailed for preserving fossils that would otherwise have been destroyed by the elements.
http://www.telegraph.co.uk/culture/film/11016945/The-curse-of-the-8-million-dinosaur.html
As I (an American) understand it, the local coroner gets to determine if the treasure was lost or hidden. If lost, the find belongs to the finder. If hidden, it becomes “treasure trove” and the Crown (usually a local or national museum) gets first choice of the objects found. But the finder is paid full value for whatever the Crown takes. Anything left over belongs to the finder to deal with as he/she sees fit. Of course the finder may have to split the proceeds with the landowner.