The $600k was calculated by the experts with estimates on Markle's most notable pieces of jewelry, as well as other reported gifts she's received from Prince Harry.
For instance, in 2019, right around the time that the couple’s eldest, Prince Archie, was born (and they celebrated their first wedding anniversary), Prince Harry gave Markle an eternity ring, which he designed in collaboration with Lorraine Schwartz. According to Steven Stone, this ring is worth around $100,000.
Then, there's Markle's engagement ring, which was made with stones from a brooch belonging to Princess Diana, and which is reckoned to have cost around $150,000, according to the gem experts. (However, some sources, like TIME, estimate that it's worth is closer to $200,000.)