I have a story to share:
I knew a guy some 10 years ago who worked at a GM garage as a mechanic, one of their top guys. He had a side business of rebuilding trannys at home at night in his own garage (more like a shop, complete with 2 hoists). This was a word of mouth operation, he wasn't stealing business from the dealership. Anyway he always had 3 trannys apart in various stages of teardown or repair at any given time and would complete one about every night ...or every other night if he didn't get parts on time. Charged $300 plus parts if you brought the tranny in (he preferred not to remove trannys). Did this for years and had it down to a pretty quick operation. This guy was making a mint, gave a warranty too. The thought came across my mind to do the same thing with manual trannys and transfer cases.

 

Once you get your first one apart it becomes very clear how they work, and they are pretty easy to repair. Manuals normally just need syncros, dogs and sliders. Automatics, it's bands, valve bodies and governors.

The trick is you often need special tools, press adapter plates for manuals, odd sized snap ring pliers for autos. I think the easiest I've ever done was a 915 from a Porsche 911, toughest was from a 90's nissan maxima. I can get most everything I need from Napa for them. Sometimes I need the local machine shop to press on and off a new gear, but it's just a few bucks.