When considering transmission replacement cost, you have to know whether you have a manual or automatic transmission. A manual transmission is only going to require the transmission and clutch components for a replacement. However, when you replace an automatic transmission, you have to replace the transmission, the torque converter and up to ten quarts of transmission fluid. Then you have to adjust the transmission shifter linkage. Cooling lines and the cooler will also need to be flushed to remove impurities. All this extra work will make an automatic transmission more expensive to replace than a manual transmission.
For more info, see Automatic Cars Vs. Manual Cars