Definitely cheaper to run a shed rather than a zed - although it may depend upon the miles you do.
Ive been fortunate to be able to run a shed this last 2 weeks instead of my 350z.
350z cost me £115 per week on fuel to get to work and back. (25mpg)
Peugeot 307 HDI Diesel costs me around about £50 per week for the same miles. (50mpg)
So whilst super unleaded is roughly the same price as diesel im getting twice the fuel economy. If I used the shed for 12 months work commuting, i would save around £3250 on fuel alone.
When you factor in the other costs involved, my zed is costing me around £900 a month in total with fuel, insurance, tax, mot, depreciation etc.. Whereas the shed works out at only £420 per month.