I can only comment on Pampers, those are the ones mandated by "the boss"...
They tend to be the same price for all packs of the same "weight", but the number of nappies in a pack decreases as you go up the size range, since the nappies are larger and less fit into a pack.
We've been on size 4 for a while now, and a good price is R2/nappy. So a larger pack with 70 nappies, I aim for R140 on special. Right now the SuperSpar around the corner has them for R156, which is close enough. (I bought a lot of 54 nappy packs when they were R100.)
We tend to keep an eye on prices at Spar, PnP, Clicks, Baby City and Dischem, and buy 4-5 packs when we see a special. As mentioned before, if you end up with too many packs of a given size, places like Dischem / BabyCity will generally swop it for the next size up.
But size 1 you don't use for long, and we've been using size 4 for many months now.