I have never done it myself, but it always looks fascinating to me. How does the cost rise to $900? Motors?
Depends what you're going for and what body kits you get, really. In my case, my Rapidpistol cost about that much because 1. The actual rewire, new motors and hydrographics were done by someone else for $710, 2. I bought a 3D printed front and rear block that cost another $60, 3. Put a holographic sight that cost $40 on it, 4. Have a $25 aftermarket magazine on it, and 5. Have a $25 aftermarket stock on it.
Of course, modding does NOT need to cost that much at all, you can very reasonably just find a Longshot, a piece of K-26, some appropriate fence cutters and a screwdriver and get upgraded performance that way. One thing I love about modding is you can put whatever you want to any extreme you want, to make something completely your own.