I’m as much a knife guy as I am a gun guy and I never go cheap when it comes to knives.
There are as many different opinions on size, type, yada, yada and often it really depends on your size and where you can comfortably carry one and certainly on how you intend to use it.
I practice and use a rather unconventional knife fighting technique in which the primary goal is to use the knife in a purely defensive manner to keep the subject back off of me while I get to my gun with my other hand.
I go blade down, edge to towards the attacker where I can if needed brace the blade against my forearm and use it in the same way you’d use your forearms to block blows.
If I have to go on the offensive with the knife I simply reverse the edge so that it rips as I make downward strikes or sidestep the threat and use it to stab and rip upwards in the area of the groin, neck, back of the knee, or underarm.
With that in mind if I’m carrying a sheath knife it’ll be a Tops C Cheetah as it suits my purposes very well and my technique as well as being a great hunting knife. They make a very similar, slightly smaller knife known as the “Sparrow Hawk”.
If a “nick knife” is more to your liking the FIEL is an excellent choice, it’s seletonized and you can decide how thick/heavy you want the handle to be simply by how much para cord you wrap around it.
I’ve used knives both in self defense and in combat and as I was trained I’ve been convinced you really need no more than a 2-3" blade for self defense. There are no vital structures of the body that cannot be reached with a 3-3.5" blade used properly. Most important features, sharpens easily and keeps a good edge, and a thick blade so that if you get it buried in the “subject” and have to pull it out at an odd angle the blade won’t snap on you.
Known your state’s knife laws at in many states any double bladed “dagger” type knife is unlawful.
Oh, I also have a couple of old Gerber “River Survival” double edged knives/water rescue knives that fall into that latter category.
You can check on Ebay or any online knife source and you’ll find all of these are in your price range as well. Tops prices don’t get too nutty until you get into the