There seriously is no "one size fits all" answer when it comes to money management.
The percentages really are (and should be) different depending on your income level.
All that aside, one thing I PERSONALLY look at it, is "What will owning X REALLY cost me". Now when I say that, I don't mean how much is the purchase price, I mean how much is the purchase price in relation to what I could liquidate it for.
The GT-R is a good example. I have caught flack from people for being "wasteful with money" by purchasing that car. One of them in particular looks at thing polar opposite as I do. He purchases EVERYTHING with cash, and tends to buy brand new vehicles with his supplier discount. The vehicle he bought a couple years ago purchase price (after discounts) was about half of what I paid for my high mileage GT-R around the same time. However, here we are two years later, and his car is worth about 12k less than he paid for it, and mine is still worth more than I paid for it. He has also been without all of his cash, and while I am paying ~2% interest on my car loan, my investments have been earning much more than that. To me, my purchase was the more prudent one... YMMV