The Doyle New York auction house says the 1992 letter sold Monday for $3,750. Its worth had been estimated at $300 to $500.
The second-priciest item was a 1991 thank you note in which Jacqueline Kennedy Onassis told Koch, "Your love of life is contagious."
The auction house says the roughly 200 letters, books and other ephemera on the block sold for $23,844, including a buyer's premium. Doyle had estimated the collection at less than $5,300.
Koch led the city from 1978 to 1989. He died in February at age 88.
