With the xception of he roofing felt, insulation and paint, it is all second hand. Reclaimed barn doors, a tatty field shelter(that was once an aviary from the New Forest), eBay doors and windows and some shuttering timber, etc.
It's my space too: I'm a textile artist and I can totally immerse myself in my work when I'm in there. Also, my Jonny built it for me (well it's almost finished, anyway).
Jonny reckons he built his for well under £1000. He was in the right place at the right time for the barn doors, for example. He says, "keep the thing off the ground; timber wont't rot if it doesn't sit in water. You don't need that yukky pressure treated stuff. Think about compacted gravel on hard core base rather than concrete. Enviro- friendlier and drains water away. Allow air circulation too.