- Plaster starts off as off-white and pigments are added to achieve the vast range of colours desired. A black polished plaster finish will work out slightly more expensive than a white plaster finish due to the extra material costs involved in turning the plaster black. - Some finishes are quicker to install and less labour intensive than other finishes and require fewer coats of plaster to achieve the end result. The less product and labour intensive the finish, the lower the price. - The substrate the polished plaster is going over needs to be in an acceptable order so the more preparation required to get it up to standard, the higher the costs. Sometimes this still works out cheaper than totally replacing the substrate altogether so it's best to discuss with us first to work out the most cost effective solution.