My rule of thumb is nothing at a flea market is worth more than $20 just because it is at a flea market. (OK, it's not scientific, but it works well for me)
There is one on ebay UK for $35, but that's BIN asking ,and it's still there, so 15-20 is probably more plausable....
Tell him he is missing the leather holster that goes with it btw ;P
There is one on worthpoint also ...(I don't have an account there, so it won't tell me what it sold for ...)
They don't sell often,so, really, it's worth whatever you are willing to part with to get it ;P
Worthpoint one with holster:
http://www.worthpoint.com/worthopedia/vintage-german-hammer-hatchet-screwdriver-d-g