Gee, Uncle Ethan, you are the only internet poster on Colt Santa Fe Special Agent that Google can find.
If it is a Colt commemorative, it is worth a bit, if...
If it is a Santa Fe commemorative made for them (the railroad?), it is worth a bit, if...
If it is one of those things you see the full page ads for in the gunzines, I think resale value is low. There are going to be a lot of disappointed heirs who get those things in their bequests.
The "ifs"; a commemorative must be new, unfired, preferably uncocked, ideally untouched, in the box with papers to be of value as such. Nice if it is still in the outer sleeve and shipping carton. If it has been used or even handled much, it is just a second hand gun. A friend bought a Colt 125th SAA factory commemorative unshot but handled for about $200 less than the price of a standard blue/CH gun at the time.