Will Steam be able to afford it?
Every item sold on steam (games, cards, wallpapers, skins in some games,etc...) is a source of money for Valve ! So YES, an Os with Steam as a user interface will make them earn money and they will be able to pay the low cost this.

And the fact is that using an os that you can customize to your needs, based on good secured, tested components is GREAT ! Just try it ! Try Ubuntu for exemple ! Try to tweak it, it's the easiest GNU/Linux distribution and it's well documented.
You will soon understand why we want to play our games with this sort of OS !