Metal detectorist Rob Brown unearthed the rare coin of Baron Eustace Fitzjohn - a wealthy landowner who became embroiled in a tussle for the English crown in the middle ages.
The 12th century treasure measures just over half-an-inch (19mm) across, was minted in York and is one of only 20 surviving examples of this design.
Rob discovered the coin on a stubble field near Pickering, North Yorkshire, after walking just 20 paces to get his first positive metal signal of the day.
The 56-year-old from Leeds found the coin in August, just two inches down in a clump of soil when he noticed the edge of a silver coin.
On closer inspection he saw it had a design on one side of a standing helmeted knight holding a sword.
Around this is an inscription which reads 'EUSTACIUS' and on the reverse is a cross within a quatrefoil symmetrical design with the legend 'EBOR. ACIT. DEFT'.
Rob, who has been metal-detecting for eight years, did not initially recognise the coin which is a very rare silver penny issued in York by Eustace Fitzjohn, the Lord of Malton and Knaresborough who served under King Henry I.
It was thought the coin could fetch between £10,000 and £15,000 at auction, but in an online auction yesterday (Tues) it sold for £29,760.
Following the sale, finder Rob said: "The auction felt quite surreal and it seemed to go so fast.
"I was amazed by all the interest from around the world with bidding from the USA, Brazil and the UK.
"It was very exciting; selling for much more than I expected. I am planning to add the money to my pension pot, and continue to carry on metal-detecting as I would love to find another one."
Eustace Fitzjohn became a wealthy landowner through marriage and later supported the Empress Matilda when she fought a civil war over who would inherit the English crown with her cousin Stephen in the period known as "the anarchy" in England.
There is a fortification which dates from the period of this conflict near where the coin was found.
Nigel Mills, antiquities expert, auctioneers Dix Noonan Webb said: "This is a tremendous result - it exceeded all of our expectations.
"Bidding came from the United States and Brazil but with two UK buyers fighting it out in the end the Eustace stays here."
He added: "Baron Eustace Fitzjohn was a Justician of the north and became a great monastic patron.
"He was to some extent, the Baron Alan Sugar of his day coming from a humble background but achieving great wealth and prominence.
"As it was a period of Civil War, Baron Fitzjohn had the authority to have coins struck in York which were primarily for local use."