The throwout bearing is a $35 part from Ford. Definetly get the Ford bearing, aftermarket ones seem to be of lesser quality. It can vary as far as expense is concerned because they do need to drop the tranny to replace it.
I would say replace the clutch while your at it if its close to being needed, as it probably is anyway, also check the rear main seal, again buy the Ford one. Have the flywheel ground, or replace with an aluminum or billet steel one too. Basically if your gonna spend hundreds of dollars to replace the TO bearing, may as well have a REPUTABLE mechanic go ahead and replace everything else. It would suck to replace the TO bearing then have to go back in a few months later to put in a new clutch.