For me, I don't play Multiplayer, so No, I would not buy a multiplayer only expansion pack at any price.
I would hate to see resources being expended on this at the cost of new and improved SP features, art work, an in- game Galactopedia, in-game Editor, standalone shipbuilder, etc.
I know a lot of players want multiplayer, but most times you lose something when you try to be all things to all people. I'd rather see this version stay true to the SP vison, and maybe add multiplayer to the eventual GC3.
That said, if you have the resources to do both, I would buy an expansion that included multiplayer as long as it did not ruin the single play balance and had lots of new goodies for the SP side.