One site you can check it Ebay, do a search for completed auctions on a like vehicle and you will get an idea as to what people are paying, willing to pay and or asking.
Or you could look up a like truck but in a F-350 and deduct 15 to 20% due to it being an F-550. F-450 and F-550 have a greater depressed and limited marketability due to demand in the used marketplace. Unless it is a custom designed hauler set-up or the needs or hauling high dollar RVs or horses....for some reason these people are willing to cough up the extra cash for a Cowboy Cruiser Truck.
Now with such low miles on this truck you may want to just price it as a equally equipped F-350 dually truck of the same class (xl, xlt, lariat)
good luck.