Logan, good knives will last a long time. A cheaply made knife will be a waste of money and useless after a few dives. The good news is that there are affordable good quality knives available new and used. I like knives that are one continuous piece of steel. So many of the knives that have huge handles end up falling apart. A good strong steel can be sharpened over and over again. A sharp point like a stiletto style knife will allow you to brain your fish without the point bending. Having a serrated edge on a portion of the knife is great for cutting or sawing through tough skin, line, gills, etc. the other thing to think of is that some knives have a "line cutter" on the top portion of the blade for a quick (sometimes live-saving) cut away of line that may have you tangled or wrapped on the reef. You don't need a big knife or blade. A small one will suffice and can be as lethal as a big one without getting in the way. They can be worn on a weight belt, strapped to the inside of your calf, or on your arm for ease of reaching. Make sure you can put a looped band on the handle so you can keep hold of it while managing your fish, gun, shooting line etc. I've dropped plenty knives to the abyss because I've not had the knife bungee around my wrist. Price and quality are not always synonymous. You can find quality knives for a decent price. Once you loose a knife you'll be glad you didn't spend a fortune for it!
The good thing about brands like Speardiver is that they are designed and used by guys that actually use their gear effectively, without the added price tag of the big brand names. But then again, if you are being mentored by local divers they may have used knives that they don't use any more that they may sell you for a decent price knowing your enthusiasm for learning the sport and waterman way of life.
This is probably stuff you know, but I thought I would share what has worked for me and my five boys who all dive.
Aloha!
Makoa