Sarah was never given money to buy a house or land for HERSELF. The divorce settlement stipulated that the house that would be bought would be put in Beatrice's and Eugenie's names. One was selected by the royal advisors, it met all the security requirements. But Sarah didn't end up taking it on as she couldn't afford to run it. It needed a large staff, so she rented for a few years, then moved back to Sunninghill. Also part of her divorce settlement she signed over her share of Sunninghill (which was in both names) to Andrew. He sold it a few years ago for a huge profit which is the source of a great deal of his current wealth. Sarah got nothing from that sale.
She gave half her divorce settlement to her mother as the ranch in Argentina was heavily in debt after Hector Barrantes's death. Sarah owns a share of the ranch, it's lived in and farmed by the Barrantes sons.
Sarah's main problem is that she is generous and is not practical when it comes to money. When her daughters were young, Andrew paid the school fees, but Sarah paid everything else. She was earning large sums of money and was generous with it, to the extent that a couple of years she even paid for Andrew's staff Christmas parties. (Before Sunninghill was sold and Andrew had less money!) It's not surprising that now her daughters and Andrew are prepared to help her out. Whether or not she's going to acquire a more practical approach to money, we'll have to see.