Each ship will require a "generator" to keep it running, so Notch's current idea is that "one subscription gives you one generator." Presumably, he says, multiple players could run around inside one ship, so you may pay a subscription and then invite your friends to join you on your vessel. But someone will have to pay to keep the game going.
Notch also talks a bit about how construction will work in the game: Players will likely build a ship's external features first, and then organize various components inside for different layouts or more options. And Notch says there will be "mining," in that players will have to find resources in the galaxy to use as parts, but he says it definitely won't be Minecraft-style. As Notch told us at PAX, work is definitely well underway, and we should see more very soon.