If it is a 1902 Singer 15 it should be in a treadle. It's possible it was installed in an electric cabinet at some point. $100 isn't completely unreasonable for a model 15 in a treadle if they're in decent shape. Mr Cheap saying $100 might be ok? I must not be feeling well.
15s tend to be a little higher priced than some models. They are a common machine though. They were made for a very long time.
Rodney
Without pictures of the actual machine it's impossible to even make a guess at the value.