Blade HQ has the HK Axis knife...
http://www.bladehq.com/item--HK-AXIS-Knife-Folder-Sand-G-10-3--27094
I can vouch for it being a great knife.
Cold Steel and Buck are getting their share of love, but there are some other options that might be just as durable and more friendly for one-handed use. Many would offer better fit/finish/materials.
The WE Knife 701 series is a beast of a knife and really nicely done. Comes in right at your budget point. Probably a better made knife than either the CS or Buck you're looking at currently.
Here's another WE, the Thraex. It's brand new so I've never had a chance to handle one but it looks absolutely awesome!
There's a whole series of Kershaw Emersons that are solid knives that land at a really cheap price.
Kizer has a lot of options under $100 that are sturdy, well built knives in both frame and liner lock.
The stainless Spyderco Endura avoids the FRN scales but brings the incredible ergonomics and awesome thumb hole of a Spydie.
I haven't used it but the RealSteel Flying Shark looks like a beast of a knife.
Some iteration of Cold Steel seems like the obvious answer, but the best thing about Cold Steel, the Triad Lock, can often be the worst thing. I've had good Triads and bad Triads. Some are real thumb busters. I wouldn't really consider any of them easy and convenient for one-handed deployment, so there are always trade-offs.
I think a lot of people overplay the lock strength of a folding knife. I suppose it's nice to have the "strongest" lock in the business, but if the second strongest lock fails on you, you're probably not using your folder the right way.
Anyway, have fun. Whatever you buy will end up being the knife you use to open the next knife you buy.