It's all relative. Depends on so many factors like how many are available in your area, how long you've been looking, how "rare" is the machine head or cabinet style. I paid 90 dollars at auction for my White twiggy cabinet VS II and I was delighted to do so. We inherited a parlor cabinet with a 115 Singer head and I paid $300.00 for a pristine, in almost new condition for a Singer 115 head with hard to find Gold Wing decals. It was worth it to me. I love that machine.
I also just paid $200.00 for a Singer 401A in a cabinet with a stool and a ton of attachments in the original hard plastic box. The machine is a gem, all the cams are there and the cam stack isn't stuck. That was definitely worth it.
We don't see a lot of nice used machines in the local charity places or on the "lists", so I guess it depends on how badly you want a certain machine.