The cards come out to $7.45 with tax (or $5.96 with annual pass holder 20% discount). It’s not the worst souvenir, although I do dislike the ability to make a custom name of your choosing for your car, but I understand that some immature people would take advantage of that. Personally, I think the driver’s license from Tomorrowland Speedway is cooler, but these are a nice way to collect your car design.
Sadly, the cards are not RFID enabled (yet), making them useless other than something you can stare at. The hope is that the cards will become RFID enabled and will actually store your custom concept cars, allowing you to keep bringing back your older designs on later visits to Test Track. Whether or not this will happen, only time can tell.