since nobody responded to you yet, i will try to give you an answer, based on my ownership of a similar Ball Fireman range...
[please don't flame me
]
there's really no definite guidelines to how much a watch is valued...price is usually determined based on condition, rarity and by willing seller willing buyer concept.
i will advise you to go to an AD selling Ball watches and check out the RRP and the max discount you can get for this model. from there, put another 20 - 30% discount so you have a rough benchmark of the price that will not be considered "too much" for a used piece.
in my humble opinion, the Fireman series is the lower end of Ball watches, so i wouldn't pay higher than 2.5 - 3k for this particular model.
best of luck and do show us the wrist shot if you eventually decide to buy it
