Six handwritten letters from Princess Diana to an ex-Buckingham Palace employee about her sons William and Harry fetched the princely sum of $18,400 (£15.1k) at auction earlier this month. This may seem expensive enough, but it's small change compared to the many millions of dollars collectors have paid for the most sought-after historic letters. Take a peek at the most valuable examples.

The leading Post-Impressionist painters penned this joint letter at the apartment they shared in Arles, France on November 1 or 2 1888, eight weeks before their friendship fell apart and Van Gogh famously cut off his ear.

A fascinating insight into the pair's friendship, the letter to fellow painter Émile Bernard reveals their dream of setting up an artist's community in the tropics, as well as the reasoning behind several well-known paintings. It realized $579,212 (£475k) at a Christie's Paris auction in 2012.

This piece of 20th-century history was sold to an anonymous Chinese collector in December 2015 at Sotheby's London, achieving four times its pre-sale estimate. The typed missive is only the second letter signed by Chairman Mao that has ever come up for auction.

In the letter, which is addressed to UK Labour Party leader Clement Atlee, the communist revolutionary requests the support of the British people for his country's “life or death struggle” against invading Japanese forces.

This 1934 letter from Lu Xun, one of China's most renowned modern writers, to magazine publisher Tao Kangde is the first million dollar missive in our round-up – surviving examples of Lu's handwriting are exceptionally rare, hence the hefty price tag.

Lu suggests that Tao, who had written to him previously for advice on whether he should learn Japanese, might be better off mastering a European language instead, in order to enjoy the great works of Western literature.

This open letter written by civil rights activist Malcolm X is up for sale on the Moments in Time website for a cool $1.25 million (£1m). The letter was discovered by chance in a storage locker and almost thrown in the garbage.

In the letter, penned less than a year before he was assassinated, Malcolm X writes about his pilgrimage to Mecca and extols the virtues of Islam. It is signed El Hajj Malik El Shabazz, the name he adopted upon conversion to the religion.

This handwritten letter from Thomas Jefferson to French geologist and explorer Barthélemy Faujas de Saint-Fond went under the hammer for $1.44 million (£1.2m) at a Sotheby's auction in 2002.

The letter, which was penned in 1804, outlines the goals of the Lewis and Clark expedition to explore and chart North America following the Louisiana Purchase land deal with France the previous year.

Letters written by America's first president, especially personal correspondence, understandably attract a premium. This specimen went under the hammer at Christie's New York in 2013 for an impressive $1.44 million (£1.2m).

Washington gives his private views on the recently drafted US Constitution in the seven-page letter to General John Armstrong, who was a member of the Continental Congress governing the country at the time.

An important piece of Irish history, the letter of surrender written by rebel leader Patrick Pearse following the Easter Rising of 1916 came up for auction in December 2016 with a guide price of $1.7 million (£1.4m).

The guide price was the highest estimate for any historic item offered for auction in Ireland. Despite its historical significance, the letter failed to meet the reserve price and was withdrawn from sale by the owner.

Considered one of the most important letters of the 20th century, this missive from Albert Einstein to Franklin D Roosevelt on the dangers of Nazi Germany developing uranium-based weapons led to the Manhattan Project and the development of the first nuclear weapons.

Einstein later regretted sending the epoch-making letter. “Had I known that the Germans would not succeed in producing an atomic bomb, I would have never lifted a finger.” It was sold at a Christie's New York auction for $2.1 million (£1.7m) in 2002.

In 2007, conservative radio talk show host Rush Limbaugh was blasted in a complaint letter from Senator Harry Reid to Clear Channel Communications CEO Mark P. Mays, after Limbaugh labeled veterans who criticized the war in Iraq “phony soldiers”. The shock jock got hold of the letter and decided to auction it on eBay to benefit the Marine Corps-Law Enforcement Foundation.

Limbaugh pledged to match the winning bid and ended up having to dip deep – the letter realized $2.1 million (£1.7) which, at the time, was the largest amount paid for an item sold in aid of charity on eBay.

This game-changing letter penned by John Hancock, the president of American's Continental Congress, is up for sale on the Moments in Time website with a hefty $2.5 million (£2.1m) price tag.

More suited to a museum or national library than a private collection, the letter, which is written in the finest 18th-century copperplate script, is a cover note for New Hampshire's copy of the Declaration of Independence.

The famous 'God' letter from Albert Einstein to Jewish philosopher and author Erik Gutkind was put up for sale on eBay in 2012. The missive was snagged by an anonymous bidder for just over $3 million (£2.5m).

Written more or less a year before his death, the revelatory letter expresses Einstein's belief that God does not exist and offers a telling insight into the great physicist's views on Judaism and religion in general.

The most expensive letter penned by George Washington ever sold at auction, this record-breaking missive to his nephew Bushrod argues passionately for the ratification of the newly-drafted US constitution.

The letter, which had been owned by descendants of Bushrod Washington for more than a century, was sold to an anonymous buyer in 2009 for $3.2 million (£2.7m) at the New York branch of Christie's auction house.

This letter from Abraham Lincoln to teacher and abolitionist Mrs Horace Mann, widow of the great educational reformer Horace Mann, is a response to a petition requesting the president free slave children.

Known as the 'Little People Letter', the missive has been described as Lincoln's most personal and powerful statement on slavery. It sold at a Sotheby's auction for $3.4 million (£2.8) in 2008, a world record for an American letter.

This groundbreaking seven-page letter from Noble Prize-winning scientist Francis Crick to his son Michael describes his “beautiful” discovery of DNA, and includes a sketch of the double helix.

The letter sold for a staggering $5.3 million (£4.4m) at a Christie's New York auction in 2013, well above its estimate of $1 million (£830k), and has been deemed the most expensive letter in the world by Guinness World Records.

Never mind Guinness World Records. The planet's most valuable letter, and then some, is this calligraphy missive penned by legendary Chinese scholar Zeng Gong in 1080, in which he muses on his recent political struggles.

Smashing all records, the letter was snapped up by billionaire Wang Zhongjun for an eye-watering 207 million yuan (just over $30 million/£25m) at the China Guardian auction house in Beijing last year.