Brake rotors span a huge price range for what looks like the same cast-iron disc, and it is tempting to grab the cheapest set. Sometimes a budget rotor is fine; sometimes it is a false economy that costs you sooner. Here is where the money actually goes.
Where cheap rotors cut corners
The biggest differences are in the metallurgy and machining. Quality rotors use a properly-controlled iron alloy with enough carbon for heat handling, are cast without voids, and are machined to tight tolerances for low runout and even thickness. Cheap rotors often use lower-grade iron and looser tolerances, which makes them more prone to warping, judder, and uneven wear - see what is brake judder.
Rust and coatings
Better rotors are coated (often a geomet or zinc finish) so the hat and edges resist corrosion; bargain rotors are frequently bare and rust quickly, which looks bad through open wheels and can accelerate pitting - see coated vs uncoated rotors. On any rotor the friction face wears clean, but the non-swept areas rusting is a real quality tell.
When a budget rotor is fine
For a light, gently-driven commuter car, a mid-grade rotor from a reputable maker is perfectly adequate and you do not need a premium performance disc. The key is reputable - a known brand's standard rotor, not a no-name mystery disc. Match it with quality pads; a good pad on a decent rotor beats a cheap pad on a pricey rotor.
When to spend more
Spend up if you tow, drive mountains, carry loads, or have had warping problems - the better metallurgy and coating pay off in heat handling and life. Whatever you choose, replace rotors as a pair - see replace both rotors - with fresh pads and hardware for the best result.