I've heard cost talked about before as a reason for why some shows don't get second seasons, and that's obviously a factor, but it's always been my contention that a big enough viewership would make money no object. I think if S:AAB (or Firefly, or Caprica, what have you) had the ratings that Cheers or Friends got on a consistent basis advertisers would gladly pay through the nose to sponsor such shows and the network in turn would gladly pay whatever it took to keep them on the air.
Sadly, that's not always the case. The original BSG never fell out of the top 25 shows, but CBS counter-programmed All In the Family
against it, causing it to drop out of the top 10. BSG's high cost and troubled production schedule (partly ABC's fault for going to series too fast) made ABC think it could be more successful in that timeslot by moving the much cheaper to make powerhouse of Mork & Mindy
against CBS on Sunday at 8. (That didn't turn out to be the case, and Mork's ratings dropped so much that the show never recovered.) BSG was killed despite doing pretty well in the ratings against some of the toughest competition on network TV: Archie Bunker.