First, let me say that I have quite a few brand new Rough Riders (or Taylor Imperials) that I paid under $10 for, and most of them are excellent knives! But the OP wanted to know about bargains found at garage sales, flea markets, gun&knife shows, and so on. I have to confess that I've so far had no luck with garage sales. But I had one good day at a flea market, and many good days at the local gun & knife show.
Picked up this Camillus electrician for $10 at the first gun & knife show I attended. It's one of my favorite knives; I think it looks very elegant and distinguished!
Got this 1936-1952 equal end Imperial for $5 at a gun and knife show. Dull blades when I got it, but recently it became one of the first knives that I consider that I've sharpened successfully (still no burr, though

).
Bought this fine condition Imperial Barlow for $10 at the local gun&knife show, and gave it to a BF member who really yearns for Imperial Barlows.
Another $10 at the G&K show that comes to town about 4-6 times per year got me this brand new Chinese-made Remington canoe that I think is drop-dead gorgeous.
While vacationing in Michigan's Upper Peninsula a couple of summers ago, I went to the weekly Sunday flea market in a little town and found all of these for under $10 apiece (the photos were taken after I tried to clean the knives up a bit):
Schrade USA 34 OT
Kutmaster Girl Scout knife (with "Dianne R." inscribed on the bolsters)
Richards pipe tool (from Sheffield, England)
Wenger Esquire (compare to Vic Classic)
Finally, here's an example of what careful shopping can sometimes get: picked up this Tinker and a matching Vic Classic for just under $20 on sale at Menard's, so that's under $10 per knife.
- GT