I guess the low maintenance cost is from the fact that they don't have a purchase price. The restorating cost can be max 50% of "new" value as I recall and when value is set to a minimum then I guess 500cr is as low as they can go.
On another topic, the stealth cars follow a certain console when they have been downloaded. I have created a second account with a different psn and the stealth/chrome cars pop up when I start a new gt5 game.