They can be fun when working well.
Condition, and specific model will dictate the trade value.
The short barrelled sac model is sought after as are the octagon barrel offerings.
I like the nickel finished units. There is a good write up on how to make them run a little better on gun test.
Value wise $250-$650 seems to be where they list. The high end ones being nib, or something fancier.
If possible try it out, or atleast run some snap caps through.