I would expect a 66 to be in excellent condition to pay $100. for it, both cosmetically and mechanically.
Be aware that the 66 is the least expensive sewing machine that Singer made. The 15-91 was the next model up, with the 201-2 being the top of the line. Both the 15-91 and the 201-2 are quieter than the 66. A well serviced 201-2 just purrs. You can usually find a 201-2 or 15-91 for the same price as a 66. All are straight stitch only machines.
Vintage sewing machines are all over the map. Yesterday, on local sites (craigslist and offerup), vintage black Singers are listed from $50.00 to $650.00, but most were in cabinets. There was even a beige 301 in a case on OfferUp for $100.
I'd suggest you learn about the vintage machines before you decide to start purchasing them. That way, you don't end up with a house full of machines you don't use. Read about them and their advantages and decide what meets you wants and needs.
jmo
bkay