big diff between the quality of chrome and polished cast aluminum. chrome covers are basically just like factory covers - just chromed. in other words, they will deform when overtightened, etc. cast aluminum (polished or not) are higher quality, generally speaking.
Bottom line, unless you have a $100K show car, I'm not sure I can see paying $500 for ANY valve covers, but SOMEtimes you get what you pay for.
Having said that, I have paid a few hundred bucks for very good condition old vette script cast valve covers. To me, nothing beats the factory hi perf stuff, it's just cool. For our ol trucks, I stick with stock, but if I WERE to hot rod one, it'd get the old vette covers.
But, to answer your question, the difference is quality between chrome steel and cast aluminum. But, it's NOT a $470 difference... Just don't overtighten the chrome steel ones and you're fine. Some prefer chrome, I tend to shy away from shiny these days - just me tho.