You never really know what you're going to find. The sales that advertise sewing machines, usually have them overpriced. Estate sales are usually better than garage sales. I think the really good deals are always the unexpected. I don't usually go to thrift stores expecting to find sewing machines, but the last time I went, I opened up what looked like an ugly plastic case and found a Singer inside.
The first treadle machine I found was at an estate sale this summer. This woman had at least 5 sewing machines and I found myself wondering, "why would anyone need so many machines?" Now, I own 4 of her machines. I paid $5 for the treadle and the others were given to me after the sale ended and they hadn't sold.
So, I don't go very often. And I don't go far out of my way to search either. Oh, and try looking on Craig's list. Look under odd spellings like "sowing". It's surprising what you'll find. I've gotten a couple machines for free, others less than $20. You have to be willing to look beyond the grime sometimes.