I would be willing to pay the price for a full retail game $49-$69 for an expansion/sequel. I mean, sure the $9.99, was really unheard of and very very cool, but maybe the investment/payback was not worth continuing?
Anyway, i would love to see more, and games cost money anyway, $49 plus would be fine for a campaign and moveable turrets. I mean, i loved HomeWorld, it was waaaaayyyyyy ahead of its time. But the graphics are dated now. Sins looks "prettier" than any space rts out there! And it would be nice to see more, but only the Devs and Stardock know what they have planned, whether they are going to continue the series, or throw in the towel.
I hope not the later, but i have to remember that for me its a game, something very enjoyable to wind down and relax after homework and school. For them, it is a business, a way to make a living, money means a lot more in those circumstances. I dont know how many "Great TV series" i have seen go under because the network decided to fill the slot with something more eye catching, not necessarily more interesting. 
Anyway, here is hoping that there is expansions and a sequel or two... or three...
in the works.
-Teal