Looks like used are $500 to $900, so figure $700. If you can get it put in for $500, that would be $1200.
Does the rebuild get any significant upgrades? If yours failed because you pushed it too hard towing or off roading, an upgrade may solve the weakness. If your AT never had its fluid changed or pink milkshaked out, a used replacement may suit you well. I don't read about that may AT failures, so I would lean towards used.