New tranny is around $7K. I'm not sure about a used tranny - maybe $2K? Just guessing.
However, keep in mind that there's no need to replace the transmission just because a car is in snap ring range. Just become familiar with Mark B's description of the symptoms. If the snap ring starts to go, don't continue driving on it; take it in and have the snap ring and transmission case replaced. MUCH less expensive than replacing the entire transmission.