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There's no realistic depreciation in the game.
But what can they do, though? IRL, cars depreciate not just because of their now inferior performance. It's also because of the update of the gagets in cars; the new one being just better than the old one, but performance wise, very similar (most of the time). In GT5 however, the only thing that matters is performance.
For starters, they could get rid of that silly "buy a car, it's worth 1/4th as much automatically even if you bought it used." That was a crappy system to use in 1998 when other games were already getting away from it, and it's downright anachronistic now.
They could also revert the car prices back to what they were in GT4.