"by the drop" is usually a low voltage term, with a price for each jack basically. I've heard it used related to commercial wiring, where each switch or receptacle is generally a single drop from a 4-square above the ceiling. I still think of drop pricing as more of a low voltage term, myself.
I do most of my work by the opening, hole, or drop. But with that said I'd only give a builder that I know a price like that. When ever I start with somebody new I never give opening prices cuz I dont know how that builder work or if he even knows what he is doing or pays right.