Transmission building is big profit if everything goes well. I did a build recently with all 3 billet shafts, a DPC reman Triple, and all of the other billet goodies in the pan and had about $3600 into it. That's buying all of the parts at full price. If you were setup with parts at wholesale cost I'm sure you could shave close to $1000 off of that total.
Now buy that same setup through a vender and it's close to a $6000 transmission. You certainly end up paying a premium for someones workmanship, experience and hopefully some warranty. It's worth it for some to know that they've got one of the best builders out there setting up their stuff. I certainly can't knock anyone for wanting to not do it them selves. But I do recommend that if you ever even though about doing it yourself, do it. It's very satisfying to know exactly what went into your trans!