Though it was one of the most promising shows of the 2013 television season, Marvel's Agents of S.H.I.E.L.D. has been a ratings disappointment. Add in the high cost of production for the series and Walt Disney's (NYSE:DIS) ABC Network should cancel the superhero-less superhero series at the end of the season.

How bad are the ratings for Agents of S.H.I.E.L.D.?

After 24 weeks of the 2013-2014 broadcast television season (through Sunday, March 9), NBC is in first place among adults 18-49 with a 3.0 rating average.  Fox was second with a 2.7, and CBS was third with a 2.4 rating average, according to Zap2It, which tracks Nielsen ratings. ABC sits at the bottom of the major networks with a 2.1 in the key demographic.

S.H.I.E.L.D. started strongly -- as should any film tied to The Avengers universe -- with a 4.7 rating in the 18-49 demo and more than 12 million viewers, according to Numbers quickly dropped however as the second episode fell to a 3.3 rating and 8.6 million viewers; by week 7 the rating fell to a 2.2 with 6.6 million viewers.

Over the last two weeks the bottom has truly fallen out with the show scoring a truly disastrous 1.85 with 5.4 million viewers March 4 and only a 2.14 with 5.9 million viewers on March 11, despite a heavily hyped episode featuring a character from the Thor movies.

What rating gets a show canceled?

There is no specific formula as to what rating numbers gets a show canceled. Some networks will renew low-rated shows they think have the possibility of finding more of an audience in seasons to come. This is true of Fox, which has renewed the low-rated but critically well-liked Mindy Project despite it only averaging 3.8 million viewers, and Brooklyn Nine-Nine, which pulls in 5.2 million, but also snagged a Golden Globe. 

Both of those shows are helped by being half-hour comedies (which are much cheaper to produce than an hour-long action series). Comedies also offer the possibility of finding a new audience once they hit syndication. That was part of how The Big Bang Theory went from minor success to megahit and what propelled Family Guy from canceled to perennial hit.

Still, most shows that average under 6 million viewers (and many that average more) get the boot.

CBS is likely not renewing The Crazy Ones (nearly 7 million viewers on March 6). Fox -- which is the most generous with its renewals -- even cancelled The X-Factor despite it drawing more than 6 million viewers for its finale.

(Editor's note: This article originally reported ABC had canceled Nashville. An ABC representative wrote to tell us, "The show is still very much alive and well on our network." The Fool regrets the error.) 

Agents of S.H.I.E.L.D. costs too much

It's one thing to keep a low-rated comedy or reality show on the air as the costs are relatively low and the potential for ratings growth at least exists. Comedies can also get longer lives with lower ratings if the network stands to profit off them in syndication. The market for syndicating dramas where the story builds (compared to stand-alone shows like the various procedurals where each episode stands alone) is limited.  

By most standards S.H.I.E.L.D. would be a bubble show. Its ratings are bad but not disastrous though the trend has been steadily downward despite stunt casting and other gimmicks designed to spike viewership. The reason the show tips off the bubble toward cancelation is its cost.

Disney has not revealed exactly what each episode costs, but The New York Times believes it to be an expensive show to produce.

"Marvel and ABC also appear to be spending lavishly, at least initially, on visual effects to bridge the gap between the cinematic splendor of The Avengers and the small screen," the Times reported. "The first episode cost roughly $12 million to make, a princely sum for a pilot. But shooting in Paris doesn't come cheap and neither does a flying convertible, Coulson's preferred method of travel."

ABC won't cancel Agents of S.H.I.E.L.D.

While the ratings are poor and moving in an ominous direction for the costly series, ABC is unlikely to pull the plug on a show it has invested so much into. The prudent move would be to call S.H.I.E.L.D. an expesnsive failed experiment and wrap up the series -- perhaps with a two-hour finale -- at season's end.

That's not likely to happen as Disney has too much invested in the Marvel universe to give up just because the ratings versus expense would lead to pretty much any other show being canceled. Instead of canceling S.H.I.E.L.D.  ABC will likely renew the series and retool it -- something that rarely works. Expect more stunt-casting (Hey it's a guy that knows Iron Man! Is that She-Hulk?), but ultimately a show that's expensive and averages fewer than 6 million viewers won't last. 

S.H.I.E.L.D. will survive the ax this year, but its long-term prospects are bleak.

My colleague Jake Mann disagrees with me. To read his take, click here.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.