I just used an app called Car Tell Auction Prices to help me buy my Audi S8.
It lists how much dealers pay for their used cars at dealer only wholesale auctions. Talk about having some serious negotiating leverage! The data base is fantastic, it has over 60,000 records from the last 30 days covering all makes it even had Ferrari's and Lamborghini's.
I was impressed and shocked to see how high Kelly Blue Book is compared to this.
Anyway, if your a car nut or buying a used car it's the best $10 bucks you'll spend.
Car Tell Auction Prices in the app store.