I would definitely pay $50 for such a film -- as long as the quality is there.
As others have pointed out, a workable script, credible special effects, decent acting = all crucial ingredients.
And as far as script/premise/character... I would strongly suggest that this be a normal everyday girl... i.e. "ordinary people in extraordinary circumstances". She should be pretty and an interesting character, and her life (or what we glimpse of it in 30 short minutes) should not be too humdrum... but please god... keep these "staging" aspects of the production within the realm of plausibility and likeability and believability. Real people. The girl next door. (She's not a stripper, for instance, or an international spy, or princess, or some bullshit) It will only make the realism of the BE seem more exciting.
HEY -- why not a 60 minute "featurette"?
Stretch it from sit-com length to a full hour drama?!?!
Of course the clothes bursting scenes are requisite, as are the slower scenes of tightness, swelling out of inadequate cups visible through clothing, complaining of discomfort, the initial period of not knowing what's going on... the slow buildup, embarassment and prudery in public, giving way to pride and exhibitionism. Please, no "Instant, Wow, Multiple, Weird" -- instead, let's have "Slow, Fast, Big" and then maybe a little "Huge, Ment" for good measure!!!
Now, for special effects -- talk to those dudes at REAL DOLL about the latex/silicone molds!!! REALISM!!!!!!!!!!!!

Also, if the actress is already a D cup, you could dress her down, starting her out as an A cup, then with selective wardrobe and binding and small bras, you could show her growing through B, C and into her natural D cup size -- where she has her realization that she LIKES THIS -- then from there you get into the prosthetics, etc. This way you take advantage of some natural fulsomeness through the first third or so of your movie.