Hi
I would suggest you keep an eye on ebay and other sites such as Freecycle, Gumtree, Freeads, etc.
I managed to pick up a 10x12ft shed for £15 on Ebay and it was a very good quality shed (so much so that hubby claimed that one for his motorbike, etc, and I had his old one which is 10x8ft for the bunnies).
Cant imagine there are bargains like this all the time but worth keeping an eye out for one.
Good luck with your search.