According to an annual report, that income in the year to March 31 was $46.5 million.
A palace source speaking to People magazine said: "We can't identify how much of it is allocated to their office." Unlike in previous years, they were also unable to break down specific costs, such as Kate's wardrobe bill.
The $6.8 million figure doesn't account for refurbishment costs at William and Kate's Anmer Hall home nor does it cover wages paid to the couple's personal staff.
While much of their major travel is covered by the countries they visit, William's week-long tour to Japan and China this year cost $163, 000 in scheduled flights and his trip to New York with Kate last December came to just under $40,000.
Prince Harry and his staff also paid $135,000 for flights - scheduled and chartered - to Brazil and Chile last June.
- nzherald.co.nz