Prince William and Kate Middleton stand on the balcony of Buckingham Palace following the diamond Jubilee Procession on June 5, 2012 in London, England. Credit: Anwar Hussein/

Prince William will have lots to celebrate when he turns the big 3-0 on Thursday June 21: His stunning, totally besotted wife Kate Middleton, his new status as an operational captain in the Royal Air Force Search and Rescue Force, a fun, busy summer and fall that includes the London Olympics and a royal tour of Asia . . . and an extra $15.5 million in the bank!

Indeed, in honor of his 30th birthday, the Duke of Cambridge will inherit his share of late mother Princess Diana's estate. Diana, who died at age 36 in an August 1997 car crash, left sons William and Harry the lion's share of her estate: stocks and shares, jewelry, money from her divorce settlement from Prince Charles, dresses and other personal belongings from her apartment at Kensington Palace in London.

Not that William and Kate, 30, live super-lavishly given their royal status: The Duchess shockingly owns only one pair of her beloved nude L.K. Bennett pumps, which she even had re-soled last summer. As for the royal fashionista's wardrobe (including many piece she unabashedly recycles)? She buys all the stuff herself!

Per royal edict, the Duchess is forbidden from accepting freebies -- although she can accept discounts on custom-made royal ensembles.

No word yet on how the twosome will celebrate William's big day. One of their favorite things to do together? "She'll order movies from Amazon for them to watch on the couch," a source tells Us.