According to this site http://www.dailytelegraph...ry-e6freuzi-1111112622574
it says that the Danes don't have to buy Mary a new house because Fred wouldn't be able to afford one. They found out just how bad things could be with the J and A divorce and so changed Mary's to give her a different arrangement so they don't have to sell any assets to fund another divorced wife.
The relevant sentences:
Days before flying to Australia for her recent tour, Mary agreed to remove passages from her original agreement that ordered Prince Frederik to pay for a "new, suitable house'' if they divorced.
A clause saying Frederik had to pay a one-time sum of money to his wife as a supplement to her new household was also removed.
It is believed the new pre-nup is a response to the recent bitter experience of Frederik's younger brother, Prince Joachim, following his divorce from Princess Alexandra.
Joachim was forced to sell a 400-year-old historical inn belonging to Schackenborg Castle, his estate in southern Jutland, to pay for a $2 million mansion his ex-wife demanded in one of Copenhagen's most exclusive suburbs.
It is also a response to the startling revelation that Prince Frederik would be unable to pay for a house, in or out of Denmark, if he ever faced a divorce.
Despite being heir to the throne, he has no assets.