Just observe all the hardware stores (advertisement) for a while.
We bought in August last year (3x4m) for €990.
For the wall thickness, pay attention to the fact that it is at least 28mm for stability reasons.
As a roof covering (wanted long-term peace of mind), I used trapezoidal sheet metal.
In total, therefore, another approx. €400 (including battens and small parts).
Then painted in the color of the house - and the gem was finished.
PS: You can prepare the base yourself. Also make sure that the floor of the hut is ventilated from below. We placed ours on 8cm high paving stones.
Best regards