According to this article, an episode of Burn Notice costs about $2million to produce. That's for between 4 million and 6 million viewers. That's fifty cents a head. Well, up to fifty cents.
Looks like Leverage gets high 3 millions of viewers in the US. And according to the most infallible of sources (posts on messageboards):
Doctor Who has a budget of about £1M per episode (which translates to about $1.7M). That's not much lower than American TV - I know Firefly had a budget in that region, and John Rogers referred to Leverage (which, I'll grant, has less FX than Doctor Who, but OTOH they have five regulars instead of two) as having a "bog-standard" $2M per episode budget.
So when budgeting air time, what do the Networks think their air-time costs? Or do they even think of it that way? I suppose if you could guarantee 6 million people watching your show, you get to spend two million dollars per episode.
But it gets weirder. Battlestar had a decent run and its ratings were lower than Firefly's. So the decision-making isn't that straightforward. We intend to learn it.