Rupert Sweeting, head of the Country Department of Knight Frank in London, said that the biggest costs of owning a country estate are the staff. He said that for a "moderate-sized" 1,500 acre spread, you'll need a butler, cook, secretary, groundspeople and cleaning staff. (Read More: Top Towns for $10 Million Home Sales)
"And for hunts, you need gamekeepers, one or two at the very least," he said.
Total annual cost for the staff would be anywhere between $600,000 to $1 million a year.
Then there all those leaky rooves and crumbling gargoyles. Everyday repairs on your estate or castle will set you back another $100,000 a year or so.
Renovations are the big ticket item. Sweeting said most owners do a major renovation after they've purchase their estates. After about 20 or 30 years, they either redo them again or sell. (Read More: The Five Largest Landowners in America)
He said many clients spend $6 million to $8 million on renovations, though some spend much more.
In the end, the total bill for keeping an estate is upwards of $1.5 million or more.
Sweeting said some buyers don't realize the full costs of owning a country estate until a year or two into ownership. Knight Frank, he said, spends a lot of time educating them on the costs and time before sales.
"It can be a bit of a shock," he said. "We try to educate them on all the costs involved."
He said owners can offset some of the costs with tax benefits, which help farm owners. But he said the high costs of owning an estate means that the largest of the country homes now selling in Britain are often selling to super-rich buyers from Eastern Europe, the Middle East, the U.S. or Latin America. (Read More: Million Dollar Winter Wonderland Homes.)
One country estate he's selling, called Bletchingdon Park, in Oxfordshire, features a classic Georgian house with 24,000 square feet and 127 acres. The price: $52 million.
Which is why today's version of Downton Abbey might be better called "the Czars of Dorset."
—By CNBC's Robert Frank