The direct operating cost of APU versus ground power may show the ground power is cheaper to operate but that may not be the actual cost if you use a ground handling agent, it is important to know what they charge for the hire and use of the ground equipment. Zürich were very noise sensitive about APUs running but their ground equipment was usually pretty reliable.
Nothing worse than arriving on stand, dark wet night in the tropics, ground power and ground air attached only for it all to drop out when the APU goes off! From an engineering point of view it probably shouldn't happen but it certainly does. Very dark aircraft warming up nicely with 400 pax still on board. There are stations where it makes sense to keep the APU running at least until the pax have disembarked.