I disagree Adam. Add $250 for the M20 cluster and another $125 each for 1st and 2nd. Then $100 for the cover over the input shaft. Often the main shaft will be stuffed too. If the M21 is a known good working box it'll be cheaper to replace the input shaft, cluster and 3rd gear. Rebuild cost will be identical otherwise. That is assuming the M21 is any good, it could be as stuffed as a 2nd hand 6cyl M20!
I won't pay more than $50 for any aussie 4spd secondhand as half the time all I get out of them is the same stuff: case, reverse, 3rd gear and extension housing. Nearlly always the cluster, input shaft, 1st gear are stuffed and then 60% of the time 2nd gear and the main shaft are stuffed.