This is an odd glitch. It seems to randomly add the price of a car to the family funds. Sometimes it's just $900, which is the price of a Sloppy Jalopy. Sometimes it's the cost of an expensive car. No reason for it, really, and I guess EA hasn't fixed it.
The reason behind the car issue is the following :
EA has coding in place to sell reserved vehicles that are assigned to sims, when they die. However, there is an error in that code, in which if an inactive vehicle is sold, the active household gets the proceeds.
So, if the user had played a neighbor at some point, reserved a vehicle for them, and then moved onto another household: When that neighbor dies, your new household gets the money from the car sale.