Quarries are usually in remote areas and the distance from processing plants is an important cost factor. Underground quarries are costlier to operate than over ground.
The larger the size of the stone, the more difficult it is to process, handle, pack and transport. Larger sizes tend to have a higher wastage (or breakage) making them more expensive.
Stone availability has more to do with the pricing then any other factor. Additionally, handling and transportation cost drops slightly for larger quantity.