I just had an M-20 gone through. The original trans cost $500 (was supposed to be good!) After 3 new gears, slider and rebuild parts, it cost me $800.
So I have $1300 in mine. Not sure you could get by much cheaper. Most so called rebuilts are selling between $1000 and $1600.
Way to much considering you can buy a new 5 speed around $2000.