Queen Elizabeth II is known for being practical and frugal, but she has one expensive hobby that she splurges millions of dollars on every year.

During an interview with Yahoo's "Royal Box," royal finance expert David McClure recently commented on Queen Elizabeth's one extravagance, which is horses. He said that her expensive hobby, which costs the monarchy upwards of $700,000 every year, is a “drain on the private purse.”

“She’s a great lover of the turf," McClure said of the Queen. "Now this is quite an expensive hobby, at one stage she had about a string of about 20 thoroughbred horses, she had three studs, she had stables."

According to the finance expert, it is quite costly to run and maintain stables of thoroughbred horses, estimating that they needed about £600,000 ($770,000) yearly by 2000. McClure added that while some of the horses raked in money, taking care of them is still a massive drain on the Queen's finances.

“I’m sure the stud probably breaks even but the normal sort of horse racing venture doesn’t so it probably is a drain on the private purse," he explained.

Just recently, Queen Elizabeth was spotted attending the Royal Windsor Horse Show, where she often enters several of her own horses at events, Express reported. Prince Philip's wife also recently came to see the double world champion dressage horse Valegro after meeting her great-grandchild, Prince Harry and Meghan Markle's son Archie Harrison.

Queen Elizabeth II has been making an appearance at the horse show every year since it debuted as a Second World War fundraising event in 1943. Each year, the show is held over five days in Home Park, which is a part of the 950-year-old royal residence's grounds. It is currently celebrating its 76th anniversary.

Meanwhile, Queen Elizabeth interest in horses began when she was just a child. She received her first horse from King George V, who gave her a Shetland mare named Peggy as a fourth birthday present. This then sparked what would become decades of collecting and raising horses.

Over the years, the monarch has become a patron of several charities and organizations focused on horses. These include the Fell Pony Society, the British Horse Society and the Highland Pony Society.

Queen Elizabeth II
Queen Elizabeth II is seen at the Chichester Theatre while visiting West Sussex on Nov. 30, 2017, in Chichester, United Kingdom. Getty Images/Stuart C. Wilson