well they dont make them any more. Zero turn mowers are the only thing NH makes now. If it is in good shape, i would figure $1000 to $1500 at the most.
There aren't many 16 year-old gasoline engine lawn tractors worth much more than $500 in my opinion. I suggest you join the Jacobsen-Ford Garden Tractors group on Yahoo. There's lots of info there and people who know these machines.