we paid £10k for a 4x5m 2 years ago. All labour included, except paint and flooring. We wnet for the apex roof which cost a lot more, but a low sloping roof would have annoyed me.
We could have got one a bit cheaper, but chose a firm with a good record, who project managed it (with some firms you do most of it yourself, and pay everyone seperately). You also didn't pay anythinhg until it was built and you were happy with it.
That's a good guarantee in a field with so may cowboys.