In terms of cost from low to high, the order is:
Cold-rolled uncoated
Galvanised
Zinc-aluminium alloy.
Pre-painted e.g. Colorbond.
Galvanised is a bit tricky cost-wise, as the price depends on the coating thickness as well as the steel thickness. The thicker the zinc coating, the longer the sacrificial protection will last. Coating thickness can vary from 50 g/sqm to 400 g/sqm.
Galvanised gives better cut edge protection than zinc/aluminium alloy. Zinc/aluminium is mainly used in roofing applications, with 0.35 mm being a fairly standard thickness.
Post-painting is usually unnecessary unless the environment is quite humid, or welding operations are involved. In dry environments, galvanised will last for over 100 years.
If painting over galvanised, an etch primer should be applied first. These have phosphoric acid and/or zinc phosphate as passivating compounds. Painting would be recommended for a marine environment.
One poster here suggested muriatic acid ( i.e. hydrochloric acid ). That creates a corrosion cell unless completely removed, and is bad news for any crevices.