To your point about high end and low end pieces of beef, steaks are usually priced by the tenderness of the beef, which tends to have an inverse relationship with flavor (of course, there are other factors as well, and some people just prefer a mild-tasting cut).

As you might have inferred by now, good burgers are typically made from cuts like brisket and chuck that are tough, inexpensive, and high in flavor. Using high-end steaks for burgers is simply a waste, doubly so for well-marbled ones.

The fat percentage would not be difficult. Any good butcher would probably be happy to help you set such a comparison up. That is, if they don't warn you that it's a waste to use that T-bone on a burger first.