There are so many factors you can't start to list them. Airframe hours, most recent check, lease rate, how old-how new.. which model.. what the going rate is on that model, what the current market trend on that model is.....
"It's not the size of the dog in the fight, but the size of the fight in the dog"
I read somewhere that is costs around 500 dollars a month to store an airplane at one of the desert sites, it cost more if the owner wants other options like if the airplane is opened every day to air out or the engines are run up periodically. Other costs would be if the airplane and engines are sealed up and prepared for long term storage.
The storage operators usually make their money when the airplane is returned to service. They charge for the inspections and repairs necessary to bring up the airplane to flight status and in most cases they are capable of doing the major inspections to the airplanes as well.
And if the airplane is not returned to service, they make money on the stripping of parts and the scrapping of the airplane. If the airplane flies out under its own power or leaves in pieces in the back of a dump truck going to the smelters, either way they make money.