To answer the OP's question, my limit is probably about $125.-$150. The Spyderco G10 Endura, Al Mar SERE folders, Benchmade Torrent, Spyderco Ethnic series (Navaja, Lum Chinese, Nilaka, Persian, Chokwe, etc.) and some of the MCusta folders come to mind in that range.
I love the philosophy of "diminishing returns", I've not thought of what this idea would be called before now, but it makes perfect sense.
I've had conversations with friends over the same kind of idea, with tools, knives, cars etc. The best argument regarding "value" is that we have to agree to disagree. One of my friends has a CRKT Desert Cruiser in AUS4, and he loves it because he can bring the edge back in a few minutes on their old worn out kitchen stone. (It'll shave arm hair.) He has a 5.11-brand knife with S30V, and it's still in it's box because he doesn't want to mess with sharpening it, when the Desert Cruiser meets his needs. Another friend laughs at me because I still use my old 1970's-80's era U.S. Schrade lockbacks, while he is constantly buying - and breaking - "gas station specials". Go figure.
I've had three full-size Leatherman tools (ST300, Surge, Chrage Tti) and I'll occasionally use my old Leatherman PST, but my old Gerber 600 I bought here years ago gets beat on regularly here, and it's still going strong. I'm probably going to sell or trade the Charge Tti here soon. I love it, but I had to blow the dust out of it last night. It's just not getting used.
Having great blade steel is one thing, but a high initial cost + spending an hour (or more) on diamond stones just to bring an edge back is not what I consider value, but that's just me. I do admire and appreciate the amount of work & attention to detail that goes into a Chris Reeve Sebenza or a Case/Tony Bose pattern, but I just don't feel the need for a knife of that quality level to get my cutting done. Great knives, to be sure, just not what I 'absolutely' need.
I think that Diminishing Returns kicks in after the $60 you can get a Spyderco Endura for. It really is the most usability I've seen, for each dollar spent, in a knife. I've spent more, but have yet to find a knife I can do more with.
The Endura 4 was the first knife that came to mind when I read the OP. IMHO, the Spyderco Endura is one of the best standards for value in the knife industry.
~Chris