To be fair, it also matters where you buy them. Prices can be all over the place - dealer, retail, and online. I've seen a difference of up to 300% between sellers of the same type. But you can also often find quality brand stuff (denso, ngk, etc.) for a few bucks more than the chinese stuff that'll last 10x longer and perform better from a few usual suspects (rockauto, parts geek, fcp, etc.). Old or cheap fuel pumps, oxygen sensors, injectors, and plugs may be functional, but putting in a brand new quality part will make the engine run better and stronger than a lower-quality part. Not to mention I've seen a lot of cheap alternators, starters, ignition coils, cv joints, and suspension components have to be changed out within a year because the parts failed again just due to a crappy build quality. That includes AutoZone brands. There are some parts I would be ok with cheaping out a little bit more than others, but it's not a rule of thumb I would recommend by any means.