As a master storyteller famed for spinning films and television shows out of the country estates and stately homes of old money, Julian Fellowes — Lord Fellowes of West Stafford — might seem a surprising champion of new money.

On the cusp of the release of the final chapter of his Downton Abbey saga, Fellowes has grumbled at Britain’s “curious double standard of hating the rich and hating anyone who has been very successful, and yet wanting it”.

“I don’t get it,” the former jobbing actor who transformed himself into an Oscar-winning screenwriter told the Radio Times. “It seems to me ungenerous not to admire people who have done very well.”

Fellowes has certainly done very well from the Downton jamboree — and he is not alone.

Six TV series and three films charting the lives of the Crawley family, their servants, friends and anyone with the chutzpah to marry into the fictional Yorkshire dynasty have also proved lucrative for the cast, production staff and media platforms as well as tour guides and those selling at times dubious merchandise. But 15 years since the first episode aired, just how much money has Downton made?

Transition to the big screen would prove a financial triumph. The first Downton film in 2019 would take $194.7 million (£144.13) at the box office — quite a sum considering a budget of just $20 million. A New Era, the second film, was released three years later and took $92.7 million — still well in excess of the $40 million production costs.

North American audiences were particularly enthralled by the Crawley family’s capers. Almost half of the two films’ box-office takings stemmed from the United States and Canada.

Will the new film, Downton Abbey: The Grand Finale, also prove box-office gold? Cinema receipts for sequels tend to be lower than originals and some of the early reviews for the new film have been what Lord Grantham might have called “absolute stinkers”.

But it would be a bold call to predict that The Grand Finale will be a commercial calamity.

Even if The Grand Finale struggled at the box office, there is little doubt that the Downton series has been the most lucrative creation of Fellowes’s career.

Tracking how much he has made from his screenwriting over the years is not possible because Fellowes’s two companies — Praemium Intentus (Latin for intended prize) and Launcelot W Films — are of a type that have no legal obligation to publish annual accounts with Companies House. These “unlimited” companies are entirely legal but are seldom seen by the team putting together The Sunday Times Rich List.

A prolific writer for television, Fellowes will have had his biggest paydays from his film work. He won his Oscar for the screenplay of Gosford Park, a country house murder mystery that came out in 2001 and now has the look of a Downton warm-up act.

Over the years, films that Fellowes has written, produced or directed have taken more than $700 million at cinema box offices. From behind the camera, star creatives in the film world can ultimately earn around 10 per cent of a film’s takings.

Fellowes’s TV work — including the historical dramas Belgravia, Titanic and The Gilded Age — coincided with the boom time for the small screen. All told, it seems unlikely that he has earned less than £50 million from his pen over the years.

Born in Cairo, home for the Cambridge-educated scriptsmith is now a grade I listed manor house in Dorset. The house dates back to the 17th century and has grounds laid out by the revered landscape designer Humphry Repton.

A peer since 2011, Fellowes’s parliamentary register of interests reveals that he owns five rental cottages in Dorset and 1,000 acres of woodland, arable and grazing land. His property portfolio also includes three Westminster flats that he rents out.

The financial winners from Downton’s success of course extend well beyond Fellowes. The Centre for Economics and Business Research estimates that tourism to the Downton filming locations has generated £172 million in consumer spending since the show began to air.

“Downton Abbey has made a clear contribution to the UK economy,” said Pushpin Singh, the London consultancy’s managing economist. “Our figure only captures ticket sales and entry fees at Highclere [Castle in Hampshire] and other film locations.

“There are also wider benefits from Downton-related tourism for the hospitality and retail industries that are harder to capture. It’s striking just how popular the programme has been with North American tourists. They are among the UK’s highest-spending visitors.”

Highclere Castle trades nicely from being cast as Downton Abbey. The 5,000-acre estate on the Hampshire-Berkshire border has been owned by the Carnavon family for almost 350 years and Fiona Canarvon, the 8th countess, has cannily nurtured a range of revenue streams.

There’s a roster of events throughout the year, including this weekend’s celebration of Highclere’s war years.

From February the countess is offering her “From Downton Abbey to Tutankhamun” tour. The £140 ticket price includes a guided visit of the house and a copy of her latest book, which explains how the 5th Earl of Carnavon discovered the tomb of the Egyptian pharaoh in 1922.

Three-day stays in Highclere’s two-bedroom Grotto Lodge are available for £1,700. Then there’s the countess’s gift shop which sells Highclere-themed teasets for £350 a pop. Downton-themed shopping bags are available for £14.50 and you can have a tea towel for a tenner.

Other purveyors of Downton “merch” may cause some pearl-clutching. Head to the website downtonabbey.shop and you will find a vast range of stationery, clothes and homeware made in the Chinese city of Wuhan.

There’s a “Noble” dress featuring Lord Grantham’s face and his line, “Don’t call me Donk”, and even a shower curtain featuring the face of Violet, the Dowager Countess, memorably played by the late Dame Maggie Smith.

The same website’s gymwear range includes a “racer-back” tank top sporting her face and her greatest hits, “I never argue: I explain” and “What is a weekend?”. Another top bears her image with the line “I do wish one would f*** right off” — words unlikely to escape a countess’s lips or ever grace a Fellowes script.

Although few Downton scenes were filmed in London, that hasn’t stopped an enterprising tour operator from laying two-and-a-half hour strolls around the capital’s Downton locations. Committed fans can pay £35 to see the spot where Lily James’s character Lady Rose scandalously hooked up with jazz singer Jack Ross or gawp at the Crawleys’ town pad in St James’s Square.

Ultimately, the real winners of Downton’s financial success lie far away from British shores. Comcast, the US media giant, controls the rights to the show through its ownership of the London-based producer, Carnival Films. This means Comcast reaps vast sums not just from The Grande Finale but from Netflix’s streaming of the 52 TV episodes.

Downton aficionados will recall Lord Grantham was only able to cling on to his home and estate by marrying Cora, a minted American heiress. So perhaps it’s not unfitting that it’s a US company that now reaps the biggest rewards from the Downton legacy.