As investors dig deeper to find high yields, they're paying unjustifiably high prices for shares of high-yielding mortgage REITs. Don't make the same mistake.Annaly Capital Management (NYSE: NLY) and American Capital Agency (NASDAQ: AGNC) now trade at a 10% premium to their last reported net asset values, or tangible book value, a price that doesn't reflect declining fundamentals for the mREIT industry.Mortgage REITs are the textbook example of a commodity business.