I guess the theft of a more expensive car should be investigated with higher priority because selling it gives criminals more money to work with and leads to more severe crime. A group that can steal and sell a Lamborghini likely runs a much larger and more organized operation than a group which steals and sells old cheap cars.
This is all guessing though, I'd love to see more data on it.
This assumes a linear margin on units of stolen cars to value. Smaller ticket items are easier to fence specifically because they are common. It's hard to sell the Mona Lisa. It's easy to sell a mass-produced TV. Cops would spot a stolen Lamborghini as soon as the APB comes in. Not so much for a Toyota Camry.
Yeah, and that's why I said that a group that can actually steal and sell a Lamborghini successfully should be investigated with more resources, since you are more likely to find a well organised criminal organisation behind it, if they can shift Lamborghinis the can probably shift drugs and guns too.
This is all guessing though, I'd love to see more data on it.