While there are many factors that influence the end price of powder coating the two below make up a good part of the variation in cost.
Type of powder
There are many different types of powder available to be used on architectural aluminium and the correct one for each application should be chosen. Use Choosing the correct Powder. (Link)
Most powders in New Zealand have been made to withstand harsh ultra violet conditions. They are normally made as an Extra Durable product that will cover a good part of New Zealand, however, as conditions get harsher, such as coastal or geothermal, then the powder type should be upgraded. Powders that are designed to withstand these harsh conditions can cost significantly more than that of an Extra Durable powder.
Powders that are not solid colours, such as metallics or pearlescents, can also be more expensive to manufacture and are reflected in some pricing.
Due to the number of powders available, including different types, colours, gloss levels, etc., it is impossible for the powder manufacturers or the applicators to stock all the variations of these. Unlike wet paint, powder has a limited shelf life and it is manufactured as an individual product i.e. it cannot be tinted from a base colour.
Typically, popular colours that are coated on a regular basis will be priced in a bracket, then powders stocked by the manufacturer but are only coated infrequently may be in another bracket. From here you can get into an area where special colours, types of powder, and made to order products may be priced. Where special powders are manufactured it is typical for all the powder to be charged out, whether applied or not, as the applicator cannot do anything with what is left over.
Some of the higher durability powders have to be baked for twice as long as some standard powders and this will also be reflected in the pricing.