The Mandalas and Thangkas are priced not on the basis of the size but the price depends on the material used to paint and the type of artist painting it.
For a glance, the same the same size of Mandala range from 800 to 4100. The difference is that the one with low price is made by a beginners painter with the basic color combination while the high priced one is made by a master with high-quality colors and probably a 24-carat gold coating.