What Hollyweird has done since the 1970s is taken low budget and b movie material and create big budgeted blockbuster movies out of them. You could clearly say something like Silent Running or Boy and His Dog, or It Came From Beneath the Sea, are cult movies.
We don't say that about tremendously popular movies like Star Wars or Jaws or horror franchise movies like Nightmare on Elm Street or Friday the 13th or Halloween because they are very popular genre films. All of these would have been Drive-In Movies, the kinds of things produced cheaply by Roger Corman and AIP. Star Wars and Jaws didn't have huge budgets for when they were made but they had sustantial budgets compared to any of the low budget horror, creature feature or science fiction movies made before. The first Friday the 13th and Nightmare on Elm Street movies had modest budgets, but these were substantially larger than the budgets used on the original Texas Chainsaw Massacre, Night of the Living Dead, or even Halloween. And the sequels they produced were made for increasingly more money, approaching medium budget movies. These end of the world epics they spend 150 million and more on now... Transformers? 2012? Spending huge money on these would have been unthinkable 30 years ago. These are the stuff of low budget and cult movies.... Not anymore.
Some still call genre movies cult films out of habit or because they appeal to a clearly defined audience that isn't meant to include everyone. You could of course say this strictly speaking about any movie rated R but we don't.
You can define a cult movie however you would like of course, but a more accurate Cult Movie is one that's not known by a lot of people--and I mean people who would be interested in it. It could also be a movie that did not do very well at the box office, but has since developed a faithful following, still relatively small in number that embrace and love the movie.
Since a Cult Movie is thought by some to mean a cool movie... some movies are marketed as cult movies --even before they are Cult Movies at all. There's a horrible movie called The Apple which has been marketed as a cult movie for many years and it's just a lousy, dull, awful movie--not fun to sit through unless you're really drunk and with a crowd (and I'd argue you could find better movies than The Apple to enjoy anyway).
Lots of Midnight Movies are I think safe to call Cult Movies. Eraserhead, Pink Flamingos, El Topo, are never going to be embraced by large mainstream audiences.