Yes, the 500 series is a slight improvement over their direct counterparts in the 400 series (580 vs 480).
Uless the 480 is dramatically cheaper, I'd opt for a 580.
Why are some 480's in the $300 range? Couple of reasons:
1. Some cards are viewed as more 'rpremium', either by consumers, the vendor or the manufacturer via their MSRP - I'd avoid those really overpriced cards.
2. That's what the seller is asking for them . It's Amazon, and some people ask crazy money. Either because they paid too much for it, to compensate for Amazon's seller fees......or simply to gouge the consumer.
I'd look to an RX580 in the $220-$250 range for an 8GB variant...