1. TM 85c tariff is a lie. Typical false advertising. It takes into account bonus airtime which expires if not used in 7 days.
2. TM 29/75c tariff can not be compared with other networks, as it is non-roaming offer. It doesn't work outside TM coverage.
Research fails on these two categories, it shows it had been sponsored by TM.
The answer (without costly research) is that the cheapest is Cell C.