I am no expert on Gravely rider values, but the biggest thing is that your tractor has the Briggs engine and not the more desirable Onan. There's nothing wrong with that engine; it's just that it's not worth as much.
I don't see any goodies like liftable hitch, front drive kit etc. With a 50" deck and snow blade, maybe $700-$1000 depending on Forward/Reverse clutches?