It is easy to build a shed quite cheaply form old doors.
On Gumtree, Preloved (There is a section called Freeloved), eBay. You can always find cheap or free doors to use. I had to take the front seat out of my car to transport . It usually involves just four bolts.
The floor can be made by collecting hardcore or bricks/slabs. Always being away freely.
I have only ever used an existing building to start, by fixing a baton to the wall. The first door is then screwed to the baton. The following doors are just fastened in place using the hinges from the doors. The only expense to speak of is the roof covering. Do not use the cheap roofing felt, it is just not worth it. The slightly more expensive roofing felt is more cost effective in the long run.
The last three images show a workbench, I got an old pine bed (Free) and some batons from a shop restoration (All free)
Doors, screws, roofing felt, slabs/hardcore.