As price goes up, the performance/cost ratio goes down. Law of diminishing returns.
I think the best performance/cost ratio is had by any knife you can find for under a dollar that will cut.
But knives are about more than performance/cost. I am willing to pay more to get more, and as a rule, doing so means the performance goes up slower than the price increases.
For various price brackets, the best performance/cost ratio knives are, IMO:
$20
Spyderco Byrd knives
Kershaw Vapor
Kershaw Storm
Ka-Bar Dozier
Buck Mayo Cutback and Hilo
Victorinox SAKs
Opinel (less than $20)
$40
Spyderco Native
Kershaw Leek
$120
Spyderco Manix
Benchmade 710
Spyderco Military
However, I think any crap knife you can get for 50 cents will offer a better performance/cost ratio than any of these.
And I got more use per dollar out of my Sebenza and Strider than any of these. I owned a Sebenza, Strider SMF, and many normal folders between $20 and $200. The Sebenza and SMF were the best, so I carried them and left the others at home. I got more for my money out of the Sebenza and SMF even if the true "performance/cost" ratio wasn't as good.
I think you just want a good knife that is quality, and you want it to be at a good price so you don't feel you paid too much. Any of the knives I listed will fit that bill and offer a lot of knife for the money.