In my area of NY state $20 is an average price. Many remain on Craigslist unsold at that price. I would never pay more. After all, they are as common as dirt....Singer made millions and millions of them. I demo quilting on a treadle at the NY State Fair every year on a..you guessed it...a 66 Redeye treadle. I paid $20 for that machine in a very nice treadle cabinet. I have a 66 Lotus treadle in my living room.
Where our country was less developed in the early part of that century, I suppose they would be more rare and more valuable....but I don't know for sure..