That's probably book value.

A used '05 S500 with 50k mi can be bought around 25K well loaded from dealers(you can imagine the trade-in value). The AMG tuning may add a bit but options are generally not valued much in a used car. LZ should take care of his cars and not switch. You may do a bit better with Lexus but not much. There are tons of low milage '05 LS430 on sale for about 30K. That's why it's a very good idea to buy lightly used cars at ~30% discount to MSRP.