Colored porcelain costs about x7 more than standard clay. The mason stains used to create the rich, deep colors are expensive and fickle. For that reason, colored porcelain pots tend to cost more than standard pots.
Another note about porcelain: porcelain is stronger than stoneware (which is already very strong) and more difficult to work with, but is considered the premium of all clay bodies. Therefore, anything porcelain tends to cost more, especially when hand-created.