Milouse, much kudos for creating such a great and useful resource for keeping track of our GT5 garages and being able to share them with friends!
I suspect you already are well aware of this, but I noticed that the fixed price for standard cars are not based on their original value, instead they are based on a very wide range of mileage, which when sorting by price, can make some standard cars "appear" significantly less valuable than others, when in fact if the prices shown were based on cars with the exact same number of miles then they would be worth considerably more.
For instance, I have been making it a point to only buy standard cars with less than 5,000 km, and have found in many cases the price I paid was over 50% more than the price as shown in your database.
This is also why we can't trade many standard cars that are "priced" just below 1m Cr., because the trade restrictions are pretty clearly based on the value of a car with 0 mileage, and not by how much it may have sold for used.
I suspect for convenience, the prices in your database for standard cars where simply based on what you and perhaps others first saw when listed in the dealership, and didn't take in account for the amount of depreciation based on the mileage of the cars, which can be quite significant depending on the age and mileage of each car.
This also got me wondering what formula PD was using to calculate the depreciation of used cars as it appears to be more complex than in past GT games.
For the various cars that I have compared price differences based on mileage, in general, used cars depreciate in GT5 by .0005% per km or for a more rough estimate .5% per 1,000 km.
This also makes it fairly easy to calculate the original value of most used cars in the dealership by multiplying the mileage by .0005% then multiplying that number by the listed price, and that will show you the amount the car has depreciated. Add that to the listed price and you'll be able to figure out the value of every used car based on the same 0 mileage starting point.
If all the prices for the standard cars were based on that, sorting by price would in fact be an accurate ranking of the true value of each and every car.
Granted, this may take some time, but all that is really needed is to put in the listed price and the mileage and then you can simply use a spreadsheet to automatically calculate the value for 0 mileage.
Keep in mind there is more to the formula than just .0005% per km, and I am still working out the specifics, but even with something as simple as .5% per 1,000km I found it very easy to determine what the original value is for each car, and by comparing to different prices and mileages for the same model, you can easily identify any adjustments to the depreciation, and thus be absolutely certain of the original value.
Regardless, your web database is fantastic, and it is great how much you have expanded all of its features and options! Keep up the great work, I am really looking forward to seeing how this all develops.