Smallish
I guess I had forgotten how small 2.5 inches actually is.
Heck, even a Case Sod Buster Jr has a three-inch blade.
I've measured a number of my EDCs and found that I don't have many with blade length <= 2.5 inches.
The medium stockman knives are all between 2.75 and 3.25 inches including the Buck Cadet.
The Case Peanut and Case Pocket Hunter come in at 2.25 inches. The Peanut can be had for around $40, but the Pocket Hunter (which I prefer) comes in at more like $50.
The first one I found that measures right at 2.5 inches is the Lone Wolf
Paul Presto, locking knife with a push-button-pivot mechanism. I can open it one-handed with an overhand flipping motion, but it takes practice. Naturally, there's no way you're going to find this one under $40. MSRP is $150, street price $100.
I have a Camillus Army/scout style knife: 2.75 inch main blade, so no joy there (oops, and no longer in production).
You might have better luck with a Wenger Evo S10 SAK; those have a main blade length of 2.5 inches, and the main blade locks. It's a very "friendly" looking knife and has all the usual SAK tools. The Evo S10 can be picked up most places for under $25.
Now, if
style isn't a problem, I do believe Kershaw makes a couple of smaller assisted one-hand openers in the 2.5 inch range. And I think they can be had in the $40 to $45 range.
Gerber has the Paraframe Mini (2.22 inch) and the LST (2.63 inch). Both inexpensive, but probably not what you're looking for.
Oh, wait -- hang on a minute -- doesn't Spyderco make a nice little compact folder . . .
. . . wait right there, be right back . . .
Found it! The
Spyderco Ladybug Salt. The 2-inch blade is made of rustproof H-1 steel. Has a thumb hole for one-hand opening. The only thing is, it's gonna run you more like $45, even at the introductory price, so if you're really set on "under $40" that could be a small problem.
Here it is in yellow.
And again in orange.
Also available kydex neck sheath.
Hope this has been helpful.