Oh... the US govt will not give a finders fee or reward or anything of that matter. More than likely, whatever parts survive the re-entry, if a person gets their hands on it and doesn't turn it over to the US gov and is found out, they will face criminal charges. This is what happened when the shuttle Columbia fell apart during re-entry. The US gov. was the owner of the shuttle, and demanded that all pieces found be turned over for the federal investigation. An amnesty period was set up and people could turn over what they found, with no questions. Well, after that period, some didn't turn the stuff over that they found and ended up getting charged with theft and messing with a federal investigation. I can imagine, the US gov't will put a far larger charge on people retrieving and keeping a piece of a spy satellite... and if someone is caught selling what they find, they may be considered treading in treason waters.
So take this to heart: if you find a piece of it, even if it is some teeny tiny bolt: TURN IT IN...