One listing doesn't mean jack crap. You have to look at average sale prices, via NADA, Kelly BB, or similar. And even at that you will notice there is a $5,000 spread between "Rough Trade In" and "Clean Retail".
Monetary Value is subjective and transitory. And it really doesn't matter how the product was originally marketed. (IE: Sky cost more because it had more standard items) That may, or may not; carry forward in time. The difference between Turbo and N/A stands a much greater chance of maintaining the spread in value, simply because of the cost to upgrade an N/A to turbo performance levels.
The automobile is the single greatest example of depreciation of value, in the marketplace. Therefore I personally consider such an expenditure to be a C.O.B. and should really be eligible for Section 179 write off.