If purchasing about all you'll be able to get for 'cheap' are cow bones (not
skulls). You might check with any local farmers... sometimes they have an
old skeleton laying out in a field being eaten away by the mice.
another place to check is any local trappers... they'll be smaller bones,
but you can get some nice skull specimens that way.
any kind of antler, large animal skull, predator skull, etc.. command pretty
large money. The boy scouts in Jackson Hole Wyoming do a fund raiser every
year auctioning off the shed antlers of the elk in the region. No idea how
much they go for.