How does one accurately reflect the real world/real market premium for ultra low miles? Well, for this car you start with the base price of $9600 retail for one with 140,000 miles. Then you factor in a realistic premium for every 10,000 miles under that. For cars in the $10K-$20K range that are 10-20 years old I typically use somewhere from $500-$1,000/each 10K miles lower. That premium is dependent on the demand for the car. The Camaro's seem to run in the $500-$700 range while the extinct Pontiacs tend to be higher at $700-$900/10K miles. I would think the Mustangs would fall within those ranges. The $270/10K miles per NADA is a joke. Probably understates mileage premium by 100%.
Will this car depreciate if you drive it a lot? Sure. It could go from $16K down to $10K with another 50,000 miles on it. But your brand new 2016 Mustang will depreciate a ton as it goes to 50,000 miles. Probably twice the depreciation of the older car....along with twice the insurance, etc.
Fwiw, if this a toy (ie 2nd car) you can add it via a collector's insurance policy for approx $100-$150 per year. That's nothing...though your mileage limit per year would typically be 3K-5K. And you can get agreed upon value in the event you total the car (ie the full $16,000 with 9300 miles).
...it's a 1998 Mustang GT. he bought it new, it currently has between 14.5-15k miles.
if he tried to sell it today, the only thing that Might get him over book, is it's a bone stock, garage kept, rust free TX car. (he Just moved back here to OH from TX, had it shipped up)
Rest assured, a 1 owner, bone stock, well cared for car like that will get well over the bogus book value. My '99 SS with 17K miles "books" for around $8600. I can find a buyer tomorrow at $11K. You will have great difficulty finding one under $12,000....despite the so-called book value. I don't care what '98-05 300+ hp car you are looking at with 10K or less miles. But, you won't find them for $9600 "book" unless you find a clueless seller going by said book.