Google charges a one time fee (currently $25 USD). Kindle (Amazon) is currently waving the $99/year fee. And they don't give any clue as to when they might not wave it.
I don't know if Apple has some sort of deal for people who only do free apps.
And of course they take a cut. Everyone does. And everyone (Apple, Google, Amazon) take about 30%.
With Apple (and I am not appologizing or excusing anything, just stating facts), they will publish your app in lots of different countries and handle any sales tax, VAT, whatever, transparently to you (for most of the popular countries and regions).
I haven't looked into the same information for Google or Amazon publishing in non-US areas.
Cheers,
Ancient Lady
AGK Community Tester and AppGameKit Master