I've just got an order (for a friend of a friend) for a timber workshop 20ftx12ft, with a pent roof, to be built out of scaffolding boards.
route a groove down the long edges of each board to be splined with 15mm ply (which I get free, but would cost about £35 a board, ripped to 1in wide - you'd need two full boards.).
9in wide scaffold boards = 70 of them to form 3 walls (set vertically).
Scaffold boards are 13ft long, so cut in half for a 6ft6 wall - 35 at £6.50 each (50p/ft from my local supplier, I'd ask for a bigger discount if I build them regularly)
another 160ft for three horizontal rails to tie the walls together + another 11x13ft for rafters = another 300ft (which could be reduced to 150ft if you ripped all the boards lengthwise.
gives a grand total of £377. + 15% wastage = £450 in timber + base + roof.
That'll give you walls of 1.5in thick timber, which nobody's going to break through anytime soon - no condensation problems, reasonably good insulation, and if you want to hang a shelf, just screw it straight to the wall.