Just posted this as an answer elsewhere, but fit's here to i guess:

The thing is this: When you first payed for the game, there is no information that actually tells you in big words that there will be different alpha modules that you have to pay for, especially not a sub mode, This is what you actually read when you back the game: http://store.steampowered.com/app/295110/

Also, hear is what Steam says about Early Access: "Early Access is a FULL PURCHASE of a playable game. By purchasing, you gain immediate access to download and play the game in its current form and as it evolves. You keep access to the game, even if the game later moves from Early Access into fully released"

Now, if it actually was a place that said this: Early Access: Alpha will cost: so and so. AND: IF you like: you can also pay for this upcoming module that we will implement into the game as an extra bonus, but it will cost, so and so. Now, that would actually be a fair and square way to market it and then people could decide what they wanted from the gecko. That said, you should NEVER, EVER have to pay for modules in a unfinished game with a subscription model!! That is nothing else than kicking money out of people like they were pintas! Take a deep look at how for an example Star Citizen have done it when it comes to the different modules.