My parents have got one in Dorset. I have no idea how much they paid for it, and it's not something I would ask either, but I know it wasn't cheap.
However, it's a bit more than a "shed". It's fairly large, even though it's packed to the rafters with deckchairs, a BBQ and the kids' buckets and spades.
Also, they get a hell of a lot of use out of it. The whole family goes down there in the summer and we stay there until the sun goes down. It's lovely if the weather is good.
You can also rent them out, but it's not something I would do personally.
You don't know what riff raff you might get in them, or how people will leave them. My parents have become good friends with the owners of the nearby huts, it's like a little hut-owner community! :)
Although it's lovely, my Mum has done it out beautifully, for me I think the novelty would wear off. I quite like the idea of a motorhome because, as Ron says, it gives you so much more freedom and choice.