I just read the "Getting to the grips with A320 performance retention and fuel sabing" where it states that reducing average APU usage by 5 minutes a day will save $10,000 a year per aircraft... only 5 minutes...
calculations made for an aircraft flying 3000 hrs a year and an average sector of 2hrs.
Now let`s suppose you reduce APU usage by 30 minutes a day... that`s $60,000 a year, by switching APU on only some 20, 15 or 10 miuntes before departure
Now will that pay off those extra cycles a year?
I think so, APUs are designed for a life full of cycles...