"the online marketplace will be developed in parallel, targeting a higher tier market than other stock photo sites like Shutterstock or Getty, with pricing ranging from around $250 to $500 per photo � a market segment which Gutsol notes has no clear leader at this time"

http://techcrunch.com/2013/08/07/500px-scores-8-8m-series-a-from-andreessen-horowitz-harrison-metal-to-build-a-photo-marketplace-expand-consumer-reach/

500px a "high end niche market"

all i've seen there is mostly about cats, dogs, sunsets, HDR cr-ap, and the typical stuff vaguely looking "artsy" you see in some Flickr groups, the only difference is the guys at 500px are in most of the case pixel peepers using the most expensive gears and of course abusing F1.4 primes.

i've many doubts this stuff can be monetized decently for the simple reason it's not art nor fine-art, go in any art gallery and see the difference by yourself, this stuff is just a step up from Flickr but nowhere near being worth 500$ for a print.

DeviantArt wins hands down in comparison and by the way there are a few famous fine-art photographers who started on DA while none is posting on 500px as far as i know.