It is $150 for the gears apparently and another $100 for the set up kit. Bearings and seals and shims and the like. Plus whatever the carrier costs.
Install on gears is $300 at the very cheapest you can find so you're looking at near $600 for the install vs a plug and play game on installing another axle with the gears already installed.
If you're looking to install a full carrier locker then going the first route is best.
Luck Favours The Prepared