Well - I could easily make a shed, but I bought one. My wife wanted a potting shed. It cost about £1,100 for a 12' by 8' heavy duty (their words not mine) "workshop", complete with shelving and workbench, proper door, fully planked timbered roof (with heavy duty felt on top), and a tanalised base beneath the floor (all on our concrete slab). This price included delivery and erection. It was a "seconds" version (which means the timber might be a bit more knotty - tghough mine was fine) I added a coat of additional preservative and beefed up a couple of areas.
It arrived a few weeks after ordering and was erected and ready to go in 4 hours.
I could have made something equivalent and saved perhaps £300 in materials, but I have so many projects on the go that my time is worth more than this. I am very pleased with it. My advice - it's a shed. Don't obsess. If money is tight, make one. If you want to spend your time on something else, just buy a decent one.