yeah, i think the 30% is fair enough, given the amount of stuff you get as a user by using steam, like

• good cross-platform support
• a working friendlist and chat system
• remote play together
• the workshop and community features
• profile customisation stuff for those that like it
• whishlists and gifts

i honestly feel like while they're a monopoly, they don't do anything other companies can't do, their cut goes to fund features others simply don't provide, so it's entierly fair for them to be more expensive than the competition