I'm in MN. Most of the old machines that are "portable" with the coffin top sell at garage sales for $25-35. About the same price for machines with a cabinet, although a treadle machine in a cabinet can be more in the $50-100 range. I don't shop flea markets, but I'm guessing prices are higher there.
I guess I would be cautious. Just because a machine is old doesn't mean it is valuable or actually good. There are many more so-so old machines than there are prized collectibles.