cheap if you do it yourself
but it all depends on the extent of the engine rebuild. just rings and a hone, or a complete overhaul: cylinders bored and/or new sleeves, new pistons, all new valve train with a re-grind, new cam, bottom end re-freshended, and so on...
you can get a freshly refurbed engine for a $1000.00 after core exchange from one of the vendors that advertises on the forum.