I have the cheap powerheads in my freshwater tank(for ~2yrs, had to buy replacements at 1yr) and have the more expensive ones in my reef tank(~3months). Two main things that have been very obvious to me so far is the quality of the more expensive ones and the flexibility of the more expensive ones.
Most of the cheaper pumps do not have an option to change the intensity of them - they're either on full blast or off. With the more expensive ones, you're able to change that setting to what you need for your specific livestock as well as put different settings into them to create varied flow throughout the day. Also, my $40 pumps need cleaned out several times a month or they clog up and shut off, where my more expensive ones have not run into any issues in the 3-4 months they've been running. Finally, even though my more expensive pumps are much more powerful, they don't stick out as far into the tank as my cheaper powerheads do in the freshwater tank, and they are mounted much more securely with the magnets rather than with modified suction cups.
I'm sure more experienced people on here will have more detailed reasoning, but that's what I've noticed so far!