Depends on what you mean by 'tons'. I have one cartridge that I switch between two cars, which required a phatbox compatible stereo in each car, and an extra phatbox. I picked up the extra phatbox off Ebay for $30 with no cable, cartridge, or anything. Then I bought a $30 cable, a $7 connector, and I soldered it up. So for $67 extra I got a phatbox in both cars, and I'd really like it if both my cars had their own cartridge, because it's annoying to take it from one car to the other.
So if you can afford 4 cars, you can probably afford it by shopping around, and you'll be able to get into it for maybe $300 per car (if the head units in each car aren't already compatible with a phatbox). If you can't afford that, you probably should sell one of your cars.
