Valve is the most famous game studio making money partnering with steam and selling games. They are also making the most money due to the games being promoted on valve/steam and if you were to work for valve it would not only mean endless amount of games but extremely good pay.
They have also introduced in game items which has only boomed the amount of money they are receiving and I believe as someone who would work for valve it would be an extremely hard job considering they also receive money doing tournaments which have end prizes of over $100,000.
But my point is that the work for the amount of pay would be heavy due to how big of a company valve is , what they do and who they do things, they not only have the most used gaming application but also sell the most off the site due to the promoting and selling of games, Valve makes way more money than any game studio out there. due to how they work with other companies.