How much money you’re getting from your parts I guess depends on the car you have.
The costs for the conversion kit also depend on the car and how much performance you want from it. There some companies selling the basic stuff (battery, BMS, inverter, motor) but the you must figure out how to put it, fabricate parts etc. So I’d say it’s a rich people hobby…
There used to be a french start-up that was building a standard conversion kit for the most sold cars, but it disappeared :(