Well, Lady Saltoun owns a stake (don't know how large) in Pictland Properties Ltd which seems pretty profitable.
There's Cairnbulg Castle (formerly Philorth):
"Cairnbulg Castle is of interest for three reasons. It is one of the oldest buildings in Aberdeenshire still to be inhabited by the family who built it, whose home it is. It is of great interest to students of mediaeval Castles and architecture, for there are many gaps in our knowledge of its history, and so much scope for study and speculation. Finally, it contains a collection of family portraits which is rare in Scotland, not because most of them are by painters of any distinction, for they are not, but because there is a portrait of every laird since 1570, and in many cases of their wives, brothers, sisters or in-laws, and this is very unusual."
The castle passed out of the Fraser family for hundreds of years until:
"In 1934 the late Lord Saltoun, 11th in descent from the 8th Laird, bought it back and modernised it. From 1966 to 1997 it belonged to Lady Saltoun, who did further modernisation in 1966. In 1989-90 extensive repairs were done and both towers, the staircase tower and the west face were re-harled. Since 1997 it has belonged to The Hon. Mrs. Nicolson, Lady Saltoun's eldest daughter. THE CASTLE IS OPEN TO THE PUBLIC BY APPOINTMENT ONLY. To make an appointment, write to her at Cairnbulg Castle, Fraserburgh, AB43 8TN."
Lady Saltoun now has a 'modern laird's home' near Braemar, overlooking 3 mountaintops. So, I'm not going to speculate on her net worth but I don't think it will be only a title that is inherited.
Lady Saltoun, 2009

and at William's wedding

Tying in to the recent change in the succession and marriage laws:
"The Honourable Mrs Katharine Nicolson, the King's sixth great-granddaughter, told The Times how an official from the Palace signed off on her wedding - in person - in 1980. She said: 'A man dressed in a morning coat was sent, although I don't know who sent him. He carried with him the register and then he p***ed off again. 'We've got a beautiful parchment document with a big seal on it, and it's signed by the Queen.'
Another bizarre situation? Mrs Nicolson told The Telegraph that when her father, Alexander Ramsay, decided he wanted a quick marriage to Flora Fraser, now Lady Saltoun, he had to put in an urgent request to the Queen. So the Queen, on a holiday on the Royal Yacht Britannia, was suddenly called into a rushed Privy Council so she could weigh the pros and cons of the marriage and then give it the royal approval. Mrs Nicolson said: 'The crucial point is that it's the law and so you have to do it. 'If someone wanted to make trouble then they could, and legally we would not be married.'"
She is a published author Lady Saltoun having written Clan Fraser – A History celebrating over 800 years of the family in Scotland (1997), a work that sold out in hardcover first edition, and whose subsequent softcover reprint in 2005 continues to be snapped up by Frasers and non-Frasers alike. It is to date the only history of a clan to have been written by the chief of the clan.
She had 2 male uncles (one a Brigadier General wounded in both World Wars) and surviving male cousins (one of whom was also a General who passed away last year), but like Queen Victoria, still managed to leapfrog them to the title despite being a female.
Fun facts:
she is godmother to Justin Welby, the Archbishop of Canterbury
her aunt-by-marriage Pamela Maude,was second daughter of the actor-managers Cyril Maude and Winifred Emery