$7k is an Ultimate transmission with all the billet parts, not the normal build needed for most of these trucks.
All the billet shafts will run $2000-3000 depending on who and where you buy them.
Now, when you start building the way DTT does on their quality units its easy to add $2000 more than a standard parts build. There are pieces DTT has developed that address the OE short comings. There also upgrade drums, pinins, etc, that add to the cost in some cases. One really has to know the pieces used to correlate costs across different builders.
The $3000 mark generally gets a solid build with the a good TC, some upgraded internals. Some places may include a cheaper billet input in that or add another $500. Going up from there its all billet shafts, billet pistons, upgraded drums, upgraded bands\clutches\steels, upgraded pinions, and specifica machining and change sto accomodate the upgraded pieces.