Have a look at how many examples of 127/128 were made. A large number of these are still around, in basements, attics etc. just waiting for the owner to pop their clogs. Demand today is limited and as Steve said they sit around for ages with the high price tag. This is why you see so many of them: It's just probability - the over-priced ones are all on display, because nobody will pay that much. Bargains last only a short time.
I've bought three model 66s in the past month (two on ebay, other at local collectables market), and paid an average of about $20. This is the head only, but all had nice decals (the nicest of them was a seized 1924 lotus for only $15). Got a 1900 Tiffany 27 head for $28 (ebay). Treadle cabinets without machines can be really cheap (current one was $16 on ebay). If you sit back a while, you'll see a bargain and grab it. Treadle belts are very cheap: Most folk think they must be hard to get and expensive (around $5), and/or that the staple is something added later to fix the broken belt.
Cabinets are extremely easy to brighten up, yet most people don't do it, and Muv's guide to cleaning and lubricating a machine head is great. Seized heads, incidentally, are a great find. The owner doesn't know what to do, and just sells it as a door stop. Half an hour after you get it home, armed with WD40, Singer oil, a few tools and tri-flow and it's been transformed into a heart pumping bargain.
I wouldn't pay more than about $100 in good condition with a cabinet, or a head only for about $20.
I'd prefer the latter only because I get a kick from rescuing and restoring.