IMO, 90% of of what you need to consider in choosing a knife for self defense has already been covered in simply deciding to carry something. The best knife is the one you have. Plain edge vs serrated, tanto vs spearpoint vs hawkbill, lockback vs linerlock...it's all irrelevant if you don't have a knife. The simple choice to carry a knife is more important than the stabbyness of a spearpoint vs the tip strength of a tanto.
Now that that's taken care of, it's time to look at the knife itself.
As fulloflead mentioned, chances are you'll be using the knife far more often as a tool than a weapon (and hopefully, and possibly never as the latter). If you've never carried a knife before, I guarantee that in a week or so, you'll wonder how you ever got along without it. Thus, I would suggest a knife that works well as a tool, not just as a weapon.
Keeping inline with my philosphy that carrying a knife is more important than the actual knife you carry, there's a few things about the knife itself that are more important than the brand, steel, or what it looks like.
Accessability. You want a knife you can get at. If it's a folding knife, make sure it doesn't snag your pocket when you pull it out. Make sure it's not so big that you can't pull it out while you're sitting or kneeling. Make sure that its blade is easy to open. Tiny thumbstuds are bad. Slippery thumbstuds are bad. A knife where you have to reposition the knife or adjust your grip a lot to hold properly after opening is bad. A knife that can't be opened easily in both a regular or reverse grip is bad. Having a knife, but one that you can't get readied is bad. Trust me, I know from personal experience.
Familiarity. Whatever knife you choose, use it. If you carry a firearm, you know all about drawing and reloading drills. The same applies to pulling out a knife, and if it's a folding knife, opening it. The best way to get familiar with your knife is to use it. Got a bill in the mail? Use your knife. Loose string on your sock? Pull out the knife. Can't get the frigging plastic off your new Jr. Walker + Allstars CD? Use the knife. Pull it from your pocket, open it, and replace it while you're sitting around watching the TV (so long as it doesn't annoy the wife too much).
Reliability. A fixed blade is always going to be the most reliable. In NV, you can carry concealed a fixed blade. If you choose a folding knife, make sure the lock will hold up. Avoid linerlocks with thin liners and ones that require a secondary lock to be reliable (yes, I'm talking about the CRKT M16). Avoid any knife where opening and locking is complicated. Something tricky might become easy enough through repetition in controlled circumstances, but in a "hightened situation", don't count on it.
I don't carry a firearm (yet), but I do carry a knife specifically for self defense, in addition to my normal, utility-oriented knives. My main folder changes between an auto (Microtech LCC or Scarab, or Severtech), a balisong (Benchmade 42MC), or a regular manual folder (Benchmade 710HS). I also carry a serrated Spyderco as a secondary knife. All are quite large/long, I'm very familiar with all of them, but none would be the first knife I grabbed for if I needed to defend myself. That knife would be the tiny, slightly silly looking Spyderco SPOT I keep strapped to the front of my backpack. Only a 1 1/2" blade, but I know exactly where it is, I can have it in my hand in an extremely short time, and I don't have to worry about opening it or if the lock is going to fail, and I know that I'm not likely to drop it or have it taken from my grasp. I have far, far more confidence in the dinky little SPOT than I do in my biggest, baddest folding knife, and far, far more confidence in my ability to have it in my hand ready to use if I ever need to. Which I hope I never do.
As for the actual knife, in your price range, I'd reccomend looking into the various Spyderco lockback FRN knives, like the Delica, Endura, Natives, and Salts. They are all very easy to open (big ambidextrous thumb holes, and very easy to flick with inertia), and are extremely useful as a tool. The lockback is very reliable and stronger than most linerlocks. The best part, IMO, is that they're so thin and light, that you can carry 2 (one plain, one serrated. This would be my preference since I dislike combo edge knives). I keep my main knife in my right front pocket, and a Salt-series Spyderco IWB. I never notice it unless I need it. Half the time I forget it's even there, and it gets sent through the wash. And they're not so expensive that you're heartbroken if you somehow lose or break it.
They're one of the original spyderco offerings, and still have a rabid following-there's a reason.
