Michael Jackson's estate

Nine months after Michael Jackson's death, his estate has signed a contract with Sony for an upfront payment of $250 million and unspecified, but reportedly high, future royalties for his music.  The film "This Is It," created after his death, grossed another $250 million. 

I have to believe the Michael Jackson estate was worth at least $500 million, which implies a death tax of close to $250 million.  Perhaps that explains the upfront cash that the estate accepted from Sony?  Jackson's heirs surely must wish he had survived to 2010.