Just for information, the last time I set a price for GPU hire, several years ago now as a handling agent, it was justifiably at least 20 X the actual direct fuel and maintenance cost, which may be the figure used in the cost comparisons GPU vs APU above.
Equipment depreciation and maintenance, labour costs, training, airside insurance, airport fees, fuel, administration overhead etc etc etc usw.
And there's the cost of keeping the back-up units round the corner in case of failure.
I'm sure someone can advise the probable contract charge for a GPU for a 30 minute turnround...my data are out-of-date. Try that against 30 minutes APU time.
But as has been said the over-riding factor is usually airport regulations, often driven by local planning agreements.