I think the advantage of bulk cost wise is you can load the number of frames you need rather than just shooting up the whole roll. I have some recently acquired RF cameras that I would prefer to load up, say, 12x just to test the basics out.
If you combine the 20x + 36x prices, the bulk is way cheaper. Then again, if only shooting 36x, then saving 17% maybe is not enough.
Its nice to have both as a choice.