dont know about cost or size, but a standard large garden shed is 8ft X 6 ft , small is 6ft X 4 ft ( but thats pretty small )
some quality garden sheds about though, treated T & G ( Tongue and Groove ) boards, the thicker the better, 15mm or 18mm
8ft X 4ft, around 1 K worth, will outlast you
https://www.gardenbuildingsdirect.co...itch-apex-shed
Going up in price and quality ( and size in this case ) 28mm log cabin style, the wall " logs " interlock on top of each other ( its very DIY'able till the roof imho ) 8ft X 7 ft, 1700
https://www.gardenbuildingsdirect.co...og-cabin/28468
I only linked to them, as I have used them in the past, they are decent enough, Ive even insulated the insides of a few and done them as garden rooms