A rare copy of the U.S. Constitution is expected to fetch millions at auction. Get an up close look

PHILADELPHIA (CBS) -- If you've got a few million dollars to spare, you could be the proud owner of a rare copy of the U.S. Constitution.

Brunk Auctions is preparing to auction off a copy of the Constitution that was signed by Charles Thomson, a Philadelphian who served as secretary of the Continental Congress and then to the Confederation Congress.

Andrew Brunk, the auction house's CEO, visited CBS News Philadelphia along with historical document expert Seth Keller to share more about the significance of this document.

When the Constitution was drafted, several copies were made so they could be distributed to the colonies for the people to ratify, Keller said. Of this run, 100 copies were printed in New York - but very few were signed by Thomson. Also attached is a resolution from the Confederation Congress to send the document to the states to ratify.

According to the listing on the Brunk website, there are eight or nine signed copies known to have survived the nearly 240 years since the Constitution was drafted in 1787, but this is the only one that is still privately owned and not in the hands of an institution.

And as for how much it could sell for? A starting bid of $1 million has already been met.

"It wouldn't surprise us that it could go for $20 million or so, it could be less, it could be more," Heller said.

Another copy of the Constitution sold at Sotheby's for $43.2 million, and "it's no more important than this," he added.

The listing says the document was passed down over generations at the Hayes Plantation in Edenton, North Carolina, on property formerly owned by Samuel Johnston, who was North Carolina's Governor from 1787-1789 and presided over the state convention that ratified the document.

"As they prepared to move it over to state stewardship, to a public property, they were cleaning things out and so on and in a filing cabinet in a storage room, out came some documents."

Further investigation and getting an appraiser involved revealed what was really on their hands.

"The sort of magnitude of the discovery came to light," Brunk said. "Imagine going through your filing cabinet and finding what is a cornerstone of American democracy."

The auction is set to take place on Sept. 28, the anniversary of the date Congress agreed to pass the document on to the states to ratify.