On YouTube just search serco. I am trying to find more informations but it's hidden.
It's possible that the Queen has less money than him. That Duke's ancestors have owned half of London for who knows how long. The Duke probably has squirreled away a lot of money, too.
They definitely could have a lot of money and valuables that is hidden from public scrutiny, but we know for sure she doesn't own the Crown Estates because that bit is out in the open.
I've been meaning to read this: https://www.amazon.com/Royal-Legacy-family-passed-wealth/dp/191019865X
Preview of the book: https://www.ibtimes.co.uk/i-tried-find-out-just-how-rich-royal-family-its-well-kept-secret-1564506