Thanks guys! My thoughts on the latest round of excellent suggestions:
Boker/Nealy: Great suggestion, and I've always admired Nealy's overall system, especially the sheath. However, I disagree about the lack of need for a guard. Or rather, I think Nealy's stuff makes more sense for a knife that has to stay concelaed, since it doesn't have a guard to get in the way. For my purposes, it must have at least a rudimentary guard.
Deerhunter: I wanted a bit more robust blade. Otherwise, it's great, especially the thumbolt sheath.
Small kabar: Well, I can't argue this is a classic combat/utility knife. However, my purposes are a little more narrow -- defensive and retention use -- so the kabar suffers from something that the SOG pup also suffers, a handle that works better for a combat/utility than a defensive knife, at least in my mind.
Greco Cub Companion: Wow! Killer find! I'm not entirely sure I want a tool steel for this knife, since it'll be left in its sheath for months at a time, but what a great deal. It's high on my list.
Frost/Ericksson puukkos: Love these knives, but guardless and with oval handles, too slippery for what I have in mind. On the other hand, don't they also have models with guards? I'll take another look at them. Deep pouch friction-fit sheaths are usually cheap, but plenty secure. I think I want more robust blades. No argument though that these are some all-around awesome knives at optimal bang for the buck.
Cold Steel Desperado: Cool knife, the pic makes me want to run away screaming

But the handle doesn't do it for me. I do have a Vaquero Grande which I like, and carry sometimes.
TTKK: Why yes, it does fill any number of roles (including a defensive role) really well, thanks for pointing that out! It is, of course, much more expensive than my price range. I do have one at home already, but it's also my most-used fixed blade knife, for just about any purpose. The reason why I'm looking for a cheap knife is that I'm going to hang it on my bump-in-the-night belt (which has the holster for the Glock, etc.). So this knife is mostly just going to sit in the closet, and may very well never be used. That's why I just want a cheap-but-functional knife. I couldn't bear to leave my TTKK or even my Deerhunter in the closet for so long!
Thanks for this round of excellent suggestions! I don't know whether or not you all like this way of the topic starter giving feedback on all the suggestions as we go along. But I always thought it'd be interesting to hear what the questioner thought of the suggestions, so I decided that next question I asked, I'd try it out.
Joe