While actual regulations vary widely from country to country, the general rule of thumb is that governments always hold the AIRLINE responsible for bearing any costs related to the repatriation of the passenger. The airline must then pursue the recovery of any costs from the passenger directly. If the passenger refuses to pay (which is almost always the case), the airline is still responsible to remove the passenger upon the instruction of the immigration authorities.

One thing of note is that most airlines require that a deportee must be carried in Economy Class. I have seen a fair number of Business Class passengers kick up a big fuss over being involuntarily downgraded when being deported. In these cases, the airline is not required to refund the fare difference between classes.