I have trouble finding any knives I like in the $60-$80 range (retail, not second hand), it seems like the hot areas for knives are $125+ or <$50.
Anyway, depending on if you can find a deal or are willing to spend a little more, these are my picks that I'd suggest:
1) Spyderco Chaparral: Can be gotten for roughly $90 second hand
2) Spyderco Native 5 FRN: I got mine for $65 before Spyderco upped overall prices last year, can easily be gotten for under $75 second hand now
3) Cold Steel Mini AK-47: Amazon has it listed for about $72, these are discontinued for 2017, so get one while you can
4) Cold Steel Hold Out series: Again, discontinued for 2017, BladeHQ was selling the Hold Out II for $70 or $75 a week or two ago.
5) Cold Steel Mini Recon 1: Discontinued for 2017, usually go for over $75, but not by much if you know where to look.
6) Cold Steel Recon 1: Can be gotten for ~$80-ish
7) Spyderco Paramilitary 2: Can be found for $80-$90 second hand all the time, one of my absolute favorites
8) Spyderco Delica or Endura: If you don't have one, shame on you, this should be #1 out of this list.
9) Kizer Gemini VG-10/G10: $80, well worth it if you don't mind chinese made, but mine came with blade play due to a loose pivot. Tightening it up to make no blade play still gives smooth action, but it's offcenter now, food for thought.
10) Ontario Rat 1 or Rat 2: Super cheap, super super cheap. D2/Carbon Fiber Rat 1's are about $60 if I recall, and the base models for the Rat 1 and Rat 2 are sub-$30, somehow, outstanding knives even if they were $50.
11) Boker USA / Boker Patriot (some places call it USA, some Patriot): ~$60 knife, pretty good specs, reminds me a LOT of a Spyderco Delica, but mine came with lock rock and wasn't very sharp, so I have mixed feelings about it.
12) RealSteel T101: $54, chinese made, not a knife I personally own but I've been looking into one for a while