I think a bike box is usually sufficient. it costs nothing. it's bigger than a hard case, but it's going to be oversized anyway, so the fee doesn't change. with a bike box, you can usually keep the rear wheel attached, which is good for the derailleur hanger and chainstays. pack it up with filler, and it should be good to go.
kind of related but not really: amtrak only charges a $20 fee for bicycles. I assume you already know this, being from california, with your fancy caltrains, pacific liners and ****.
I've also used greyhound (on my last tour). never again.
is this about work related travel, or are you trying to make it to some races on the national calendar?