I have a few friends with whom I'd like to play this game in multi, so if you add coop in there (ie, two or three human players vs a bunch of Intelligent AIs), then yes, I would like multiplayer.
However, I wouldn't pay 20 bucks for a strict multiplayer expansion pack.
If you add more content, then it becomes a different story, but it would depend how much content. As I said somewhere else, I'll pay 10 bucks for a ship package, so I'd probably pay 20 for a ship package, multiplayer and a few other (minor) things.
20 bucks for multiplayer alone? No.