The 2,500-year-old item was acquired by a rag and bone man who handed it on to his grandson, John Webber, believing it was made of brass.
As a child Mr Webber took pot-shots at the 5.5ins tall cup with his catapult because the two similar female faces on it reminded him of his schoolteacher.
Over the years he either stored it under his bed or in his loft with other “odds and ends”.
It was only when Mr Webber, 70, from Taunton, Somerset, moved house last year he remembered it and had it analysed.
He was amazed when experts dated the cup back to the Achaemenid empire, making it older than the Holy Grail.
Some estimates suggested that the rare cup would fetch hundreds of thousands when it came under the hammer at Dukes Auctioneers of Dorchester, Dorset.
But once the buyer’s premium was added the cup was bought by an unnamed purchaser for a total of £65,725.
Nevertheless, Mr Webber, who was at the auction with his 20-year-old daughter Kate and son George, 17, said he was delighted with the price and was planning a celebratory meal.
He said: “I am very pleased the cup has sold. We as a family have already decided we are going to split the money between us because, at my age, there is not much I need. I imagine my grandfather would be very pleased with the fact the cup is going back to Somerset.”