They are crazy expensive here too. I looked into getting one, for a 10x20, $200-250 a month is about average. I quickly ruled out that option. For comparison, I can rent a nice duplex that comes with a garage for about $800 a month.
Holy schmoly batman, that's outrageous. My son stores two cars every winter for $30 a piece/month - non heated warehouse. He can get at them for the entire season.