Around here treadles are more looked at than used these days. The condition of the cabinet is going to be a bigger factor than the machine most likely. Restoring a cabinet is a good way to suck the profit right out a machine. Missing parts are another deal breaker. Singer 66's are good machines but common as dirt in my market.
Your best bet is to watch Craig's List until you get a good idea of what the asking prices are for the machines in your area.
There really are just too many variables to give you a price on any machine.
Rodney