In Southern England, I asked around and found a local farm with a good barn and dry area to store my Ducati for £5 a week. That sounded quite reasonable to me. If you're willing to settle for the country, it'll be a lot cheaper. Space is very much at a premium in England these days.
The reason why I had to opt for the barn, is that my friend, who was storing the bike for me, had to give up his workshop because they trebled the rent. He decided to take his business, setting up shop at home, and unfortunately was no longer able to store the Ducati.
Better to be an idiot than a crook.