I would assume marble is more expensive than most alternatives.
Marble is easy to carve and hard to shatter. It can take and hold detail quite nicely. These considerations make marble a good sculpture stone even if it is not as easily available as other types of rock. One should also bear in mind that while most of what survives to us of ancient sculpture is marble, but a great proportion of ancient sculpture was actually done in bronze. Bronze statues, however, can be melted down and their metal put to other uses and as a result far fewer of them survive to the present day. This creates the perception that marble was a more universal medium than it in fact was.