games will be sold full on itch.io, not per update. Early access means you buy the full game, including future updates, just at a lower price because the game is still being made and hadn't been released in full.
Prices will always be the same for itch.io and steam (with very little variation cause Steam prices are set regionally, itch globally). ie, full-release prices will be same on itch and steam. And for future games, if steam gets early access, it'll be same price as itch.io
though, I should mention, itch.io has cracked down on adult creators earlier this year and just saw from other devs they did it again today. itch will probably do so again in the future. It's unlikely I'll be using itch permanently, but I'll continue using it long as I can.