The commercial success of Falling Skies seems to suggest that there is still an audience for sci-fi but just not 'space opera.'
Or just that cable is a friendlier environment to any sort of niche programming.
Remember,
Falling Skies and
Terra Nova were both getting around the same audience size - 6M or so. One is a bit hit for TNT, the other is cancellation bait on FOX. And then there's runaway hit
The Walking Dead - 7M, which is insane numbers for AMC.
There's no one "right" or "wrong" approach or audience size. It's all relative to where the show lives. As for the budget, I'm no more impressed by
Terra Nova's dinos than
Falling Skies skitters & critters, so I hope the former wasn't a lot pricier than the latter, or FOX was not getting their money's worth, but I wouldn't be surprised if that were the case.
I can envision space opera surviving just fine on cable somewhere, getting at 6M or so level, even less on premium cable. You probably want to steer clear of "childish" elements like blue and green aliens, and overly pat stories, stuff that just doesn't mesh with cable audience's expectations.
A space opera version of
Game of Thrones, with complicated political and personal stories set amidst galaxy-spanning empires, with lots of sex and violence, could work. So, what we're looking for is basically,
Dune. (Or another space opera book series with that kind of notoriety and following, I'm sure there are several.)
Another possible template is to keep it close and personal instead of grandiose and epic - revive
Lost in Space, or that kind of story, with a small group built around a family unit attempting to survive on a cruel world filled with nasty critters...that's basically
The Walking Dead in space.
Unfortunately, none of this sounds very much like
Star Trek. Taking a risk on space opera is a tall enough order, so most likely it will use inspiration from other sf/f shows that have been successful. Genre shows as a whole are doing pretty well, they just don't involve outer space.