There are many ways to skin a cat, if you can hold the bugger still. It would only take 2 tons of concrete, with a fair bit left over at an approx cost of £200 including mixer hire for a 4" depth. Anything else like sleepers would cost much the same plus the cost of concrete foundations to rest the sleepers on to guarantee a perfect level. There are more ecologically sound bases which would cost more.
In short, if you are spending serious money on the shed, concrete.