Per Celebrity Net Worth, Princess Diana's net worth at the time of her death was a whopping $25 million. After her death, her assets were put into a trust for her two children, who were both under 18 at the time — they were to inherit their share of the trust when they turned 30, at which point they ended up with around $40 million each. 

However, it wasn't just money that the young princes inherited; they were also left their mother's wedding dress and some jewelry too, while William inherited her precious gold Cartier watch — a gift to Diana from her father (via Daily Express).

Had she not have married into the royal family, she'd still have been secure in her finances. According to the Daily Express, some of her family members — including her brother Charles — are worth more than £89 million, the equivalent of more than $120 million.