The lump-sum fee enables the firm to capture all the consumer surplus and deadweight loss areas, resulting in higher profit than a non-price discriminating monopolist could manage. The result is a firm which is in a sense allocatively efficient (price per unit is equal to marginal cost, but total price is not) - one of the redeeming qualities of price discrimination. If there are multiple consumers with homogeneous demand, then profit will equal n times the area ABC, where n is the number of consumers.