Considering the amount of money the Thor films and Avengers pull in, paying Hemsworth $500,000 is a "pittance" when you're talking about that scale of blockbuster money. RDJ is allegedly the only member of the Avengers cast who was paid over a million (and estimates place his total pay for it at around $70 million, because he negotiated an amazing deal after the success of the first Iron Man), and it's said that Evans and Hemsworth were only paid around $250,000 for Avengers, and it took RDJ throwing some weight around to even get Marvel to up their pay beyond that after the movie made a billion dollars.
Yeah, on one hand there's an aspect of "suck it up, you make ten times what we make" with this, but on the other, being Thor is very much a 24/7 job for Hemsworth, because he has to look like Thor. If a series is doing phenomenally well, that should be reflected in the stars' salaries in a proper ratio. The actors are where the proverbial rubber meets the road, after all.