By Ilaina Jonas

NEW YORK, March 3 (Reuters) - U.S. real estate investment trusts, publicly traded real estate companies, are dirt cheap when measured against privately held real estate, bonds or stocks, Green Street Advisors said in a report.

"I don't think they're table-pounding cheap," Green Street Chairman Mike Kirby said in the report, released to clients on Monday.

"I think they finally crossed through the threshold where we're willing to call them cheap. It's amazing what a 50-plus decline in stock prices will do for valuations."

But the sector is not exactly a bargain. The companies in it need to reduce their debt by about as much as they are worth in the stock market, said Green Street, an independent research, trading, and consulting firm.

Since REIT shares peaked in February 2007, the sector is down 75 percent, as measured by the benchmark MSCI U.S. REIT Index .RMZ and 64 percent since last September.

Industry leaders have all seen their stocks battered. These include Simon Property Group (SPG.N: Quote, Profile, Research), Vornado Realty Trust (VNO.N: Quote, Profile, Research), Taubman Centers Inc (TCO.N: Quote, Profile, Research) Macerich Co (MAC.N: Quote, Profile, Research), Developers Diversified Realty Corp (DDR.N: Quote, Profile, Research), Kimco Realty Corp (KIM.N: Quote, Profile, Research), Equity Residential (EQR.N: Quote, Profile, Research), Boston Properties Inc (BXP.N: Quote, Profile, Research), SL Green Realty Corp (SLG.N: Quote, Profile, Research), AMB Property Corp (AMB.N: Quote, Profile, Research) and Host Hotels & Resorts Inc (HST.N: Quote, Profile, Research).

REITs are the cheapest they have been since the 1998-1999 REIT bear market, relative to other investments, Green Street said in the report.

Relative to privately held real estate, REITs are trading at a 45.3 percent discount to the value of property held in private hands and their cheapest since 1993, the start of the modern REIT era.