The marginal cost for new episodes will still be less than for new SGU episodes.
How do you figure that? Why would new episodes be "marginal" in terms of cost? It's a live action show with a large cast, numerous sets and large production staff. I don't see how the cost diminishes over time.
And keep in mind they have to recoup the $100 million-plus that has been spent to produce this turd originally. They don't start making a profit until they've crossed that threshold, and even then it won't be worth it if they only make a few million in the end.
Now, granted, I don't know what kind of numbers this show is getting in foreign countries. Maybe in the long run the collective ratings and DVD sales of this show will put them over the top, but that is a hard mountain to tackle.
And regarding your SGU reference, keep in mind that the show came with a built-in audience. DEFECATION did not. If they're starting to spend less on filming episodes of DEFECATION that's only because the investors are getting wary and don't want to risk more money on something that is clearly not succeeding as well as hoped for.