Please post some photographs of the vise.
Pay particular attention to mounting bracket, as this was one major area where vise designs evolved.
Without a photo, my best guess would be 80 - 120 years, leaning to the newer side.
The basic post vise has been around for hundreds of years and I have personaly used one that is over 200 years old.
The only thing that seems to ware out is the threads on the clamp screw.
In general:
Older brackets tend to be U shaped afairs that flair out into two wings that bolt to the table. The bracket is secured to the vise with a key and wedge system. The mount bracket also secures the spring that opens the vise jaws.
Later brackets often use a two piece arangement where the U bracket attached to a cast bracket with the companies logo and was tied to the vise with a key and wedge system.
The newest brackets are just U bolts that clamp a bent an punched mount bracket to the vise.
Other variations exist, but are not as common.
Other features that help date vises.
Look at the leg, is it forge welded at the hinge joint, or are the hinge sides rivited to the leg.
Is it an open die or closed die forging? (is the forging perfectly symetric, or can you see the hammer blows from the trip hammer when you look closely)
Does it have a set of pipe jaws, or at least the cut outs for them (newer design)
Value tends to be reflected in size and condition. 50 to 80 lb vises are probably the most common. Small ones are popular with blacksmiths that do public demonstrations and are less common. Large ones (over 100 lb) are also comand a bit of a premium.
Personaly, I have about half a dozen of them and gave no more than $15 for any of them, and have some smaller ones and some larger ones. Some needing more TLC than others.
I have not been in the market for one in about five years. Back then if you can find a complete one with a good screw and nut for $40 to $60 at a garage sale you are doing fine, but thats Oklahoma. Add a few bucks if you actualy have the pipe jaws. They are almost always MIA.
The low price reflects the fact that they just dont work that well for most shop work and they are relitively common. They are designed for forging hot iron and carying hammer blows to the ground. The jaws swing in an arc and are only paralell in one position. Most also have smooth jaws, no teeth. They will take a heck of a beating for years and years and never flinch, but try to hold anything round in them twist it and they fall flat. The lack of side to side stiffness and jaws that are out of paralell is a poor combination for that type of work. Hammer, chisel or file and they do fine.