Surprisingly, I've found that frequently (not always) the cheapest tools are the ones that last the longest.
But generally, I do buy cheap tools. If you have a Harbor Freight near you they are a good source. Sears used to sell a great line of stainless Craftsman screwdrivers and those were probably the one expensive tool I'd buy over and over. But they don't carry them any more, so now I just use any screwdriver I have at hand. A couple times a year I'll go to the local flea market and shop around for good brands. You can get a bag of Craftsman, Stanley, Snap On etc. for less than fifty bucks. That's probably where most of my hand tools come from now.
I don't maintain my tools in any way, shape or form. I know divers that spray everything down with WD40 at the end of the day, but I don't bother. And again, I've got three dollar pliers and fifteen dollar pipe wrenches that have lasted for years. I think I've been using the same claw hammer ever since I started the business. So, I don't waste too much time taking care of the cheap stuff.
Most of the time I work out of a 13' inflatable boat. I have a set of three rectangular milk crates, one holds my compressor, cords and adapters, one holds my weight belt, hoods, gloves and leashes and the other my scrubbers, scraper, suction cup and tool box. I built an elevated platform out of PVC pipe that lives in the bow of the inflatable and holds the milk crates securely and leaves room for fins, zincs etc. underneath it. When I'm not in the boat, those same three milk crates stack neatly on my hand truck and I can easily wheel my gear around. After 15 years in the biz, you learn what you will need to bring for any particular job and to anticipate what you might want to have with you when diving new boats for the first time. But even now, I'll forget stuff. That's comes with the territory. You just try to minimize it as much as possible. Because you're right, it's a major PITA to have to go back to the truck because you forgot you were going to replace a five dollar zinc on the boat you just spent 15 minutes walking to.