The two main factors that influence the price of anodising are:
Thickness of the coating
Colour of the anodising
The thickness of the anodising is directly related to the corrosion protection required. In non-aggressive environments 12 micron may be adequate, however as the environment becomes more aggressive then a thicker coat of 20 or 25 microns would be required.
Click here to go to Choosing the correct Anodising (Link).
Anodising is an electro-chemical process where the time in the anodising tank is directly related to the thickness of the coating i.e. 25 micron takes twice as long to process as 12 micron.
The natural colour of anodising is silver but by adding 3 more processing steps to incorporate a colouring system most shades (from a very light champagne colour through to black) can be achieved. Because of the extra processing time and the addition of chemicals for colouring, these coloured finishes will be more expensive than a natural silver finish.
Click here to go to the Anodising Corrosion Zones map. (Link)