what no one here seems to get is this isnt a simple process. they have to digitize the stock diff, design the replacement in solidworks, put it into mastercam, design the tooling and jigs to hold eveything for the machining process and then theres initial testing and r&d, the person that does the testing is normally the person the requested the unit in the first place. they test lock up, a bunch of stuff. and for this normally they give that first person the unit for free.

i used to work in a machine shop that did things like this. mostly suspension pieces. its a very lengthy and time consuming process.