Steam charges a one-time $100 fee per game, so it’s still similar if upfront publishing costs are the issue. But certainly that $100 is less worth it if there are hardly any Mac players and thousands of Windows/Steam players.
There's a big difference: The Steam fee "will be recoupable in the payment made after your product has at least $1,000 USD Adjusted Gross Revenue for Steam Store and in-app purchases." https://store.steampowered.com/sub/163632
