I am pleasently surprised that $1.5 million buys an elderly 737, however taking that figure and say 8% interest, thats $328 a day.
An airplane costing $20m would cost $4383 a day in interest.
I assume repayments have to be made, and depreciation allowed for, so the capital cost differences between a $1.5 million and a $20 million airplane are not insignificant, and there are probably more potential risk takers (financial that is) in the $1.5m range than the $20m range.