I built my Japanese Tea Room inspired shed out of recycled materials and skip finds, as well as stuff given away free on Marketplace. A lot of the wood came from an old playhouse, rotten stuff cut out and replaced with pallet wood and old fencing found lying discarded outside strangers houses. The uprights were discarded fence posts left over from when a neighbour replaced a fence. The only things I've paid for is screws, preservative and paint.
The shed is an ongoing build, although mostly finished. The interior is deliberately sparse, just some cushions for sitting on. The shed is very small (7X5) and is intended as a place to drink tea and relax. But I do still add bits and pieces as and when inspiration hits. It is already well used by my family and is also well liked by my two cats!