How much? Well I suggest you prepare to bend over and take some lube with you Seriously it was expensive when I checked last January for the RV. For a 25' spot it came to something like $320/MO and there was a waiting list.
I tried the one on Carefree hwy by Cave Creek and the one off N Scottsdale Road up by Pinnacle Peak I think they are co-owned). There used to be a an old outside dirt place in an equestrian area off of Lone Mt Road, but even they wanted $250
I am sure you can find better. Maybe Pleasant or Barlett?