How much does the conversion cost?

Depends on what kind of system you get. You pay for convenience. If you want to do it all yourself and you are dealing with a car, you can start as low as a few hundred bucks. But that means you'll be doing a lot of the filtering by hand, pouring grease over filters, trying not to get it all over you. To get a nice system with pumps and have someone do the work for you will cost you about $2000 for a car, $5000 for a bus.