I'd happily pay around $10-$20 for an expansion, although at this point it's hard to say what features I'd want that won't likely be included in 1.0
Mainly I'd want:
Backpacks/carry more than 1 weapon type per colonist. (Not using more than one weapon at a time, but being able to carry both a ranged & a melee weapon would be super handy.)
Surgery for pets: Being able to give my dogs replacement limbs or cyber eyes would be especially handy.
Ground ore nodes: Basically sections of flat ground that have minerals you can mine, and once mined out they turn into patches of rock or dirt. This would make colonizing flat lands much more viable.
Feature I'd most like to see:
Romance/off spring for colonists.
Unfortunately I rather doubt this will be implemented given the fact children colonists would be treated the same as adult ones, meaning they could be killed, enslaved, cannibalized, etc.. which would no doubt draw an inordinate amount of negative publicity and flak from the weenie anti video game violence crowd. (The same group of morons that got up in arms over Fallout 2 having killable children, and push for the govs to ban hammer games like Saints Row IV so they never even get released in some countries like Australia, Germany, etc.)
Feature I don't want to see:
Multiplayer: The game's not really suited for it, although I can see how it could potentially be fun raiding someone else's base, it's nothing I'd ever play personally, so I could care less about the time and work that would go into adding it and would rather see it spent on an aspect of the game I'd find more fun.