I think I spent about $500-600 in parts the last time I rebuilt a j-type.
Just the clutch was about $250 (maybe that was a d-type), add bearings, seals and it adds up. If the planetary gear is worn, it cannot be rebuilt easily and may have ruined the anulus. Solenoids aren't cheap either. Yea, it can get expensive, and the total for the parts may be more than the gearbox is really worth.

For me, I wouldn't risk more than 100 bucks on a M41 that I knew was bad.
Those units are pretty durable though and simple too. I'll bet the odds are with you that you could piece together at least one good one from the ones available.

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'60 544, '68 220S, '70 145S, '72 144E, '86 745T