Your question is actually very difficult to answer.
There are quite a few variables. Actual appearance of the unit, mechanical condition, total hours on the unit, options (front hydraulic ports, 3 point) implements if any and their condition, your location, etc, etc.
I have seen unrestored 782s in nice condition with decent mower decks, sell for anywhere from $600 to $1000.
However the biggest variable of all, is how much will somebody give you for it....and that truely sets the value of the tractor. (besides sentimental value)