A hoard of more than 52,500 Roman coins found in a field in Somerset has been valued at £320,250.
The coins were discovered in April by Dave Crisp, from Wiltshire, who will share the sum with the landowner.
Mr Crisp said: "I'm very, very happy with that. I think it's a fair valuation."
Somerset County Council hopes to buy the hoard, one of the largest finds of Roman coins in Britain, for display at the Museum of Somerset.
The Treasury Valuation Committee was responsible for determining a value for the find.
The council's heritage service has received £40,250 from the Art Fund towards the appeal.
The charity will also match every pound donated by the public up to the value of £10,000.
Steve Minnitt, head of museums for Somerset County Council, said the Frome hoard was one of the most important discoveries made in Somerset in recent years.
He said: "As such, we feel that it's a find that really needs saving for the people of Somerset, so we're launching the campaign now."
The Somerset Archaeological and Natural History Society has also donated £10,000 to the fund.
Under the Treasure Act, the local authority has four months to raise the money, although this deadline can be extended if necessary.
Mr Crisp, a hospital chef, intends to buy a new car with part of his reward money but intends to continue working until his retirement in August 2011.