B films are inherently low budget but far from every low budget film is a B-film. To be, a B film denotes a genre film that is poorly made and innately campy as a byproduct of it's budget. Thus why I wouldn't view Death Proof or The Substance as B films even if there are elements of homage to them within (especially with Death Proof). To me, the Substance is far more influenced by the aueter work of Cronenberg than it is tradition B films. Her other film Revenge, though, is very much influenced by many of the same low budget B films of the 70's that QT is.