How about it has to be repaired, so you have to go find the parts, which are scattered throughout the map, so that way you have to buy the parts as well as the doors to get to the parts. Kind of like a simpler Easter egg. That way it's not to hard to get, it just takes a little while and a bunch of points to get. Then after you finish repairing it, you have to pay so much to use it. But the catch is, is that you can only use it once or maybe a few other times before it runs out of gas or explodes or something cool. That way it doesn't make the game to easy.
Or another idea is, is that the price of the vehicle slowly increases each round by a certain percent by each round or every few rounds. That way you can still use it for a quick escape from a tight but it'll cost you. And usually by that time, that's the time where you've been down once or twice, so you need to buy your perks back, and you lost your upgraded ray gun because you had mule kick....you know? That same old story. lol