Here are relevant excerpts of the actual law which would seem to indicate that:

1. The seats are provided for free (with no tax benefit - although the tax code may have something added, and I will look further)

2. Deadheading FAM's are entitled to free travel (but space available) to get home

This is a direct cut/paste (nothing changed) from the Aviation Security Act:

‘‘(4) shall require air carriers providing flights described
in paragraph (1) to provide seating for a Federal air marshal
on any such flight without regard to the availability of seats
on the flight and at no cost to the United States Government
or the marshal;
‘‘(5) may require air carriers to provide, on a space-available
basis, to an off-duty Federal air marshal a seat on a flight
to the airport nearest the marshal’s home at no cost to the
marshal or the United States Government if the marshal is
traveling to that airport after completing his or her security
duties;