Incredibly minimal expense. He’s using OPM like a master.
Trump has his own Rules as spelled out in "The Art of the Deal"
http://www.businessinsider.com/donald-trump-business-philosophy-from-the-art-of-the-deal-2015-6
BTW, "The Art of the Deal" was first published in 1987 and is still in the Amazon top 200 in book sales.