I know mate, building prices are scary. I completed mine for around £2.5K, size around 7.5 x 3.5 mt. But I did all the work myself with some help from a builder friend of mine who. The only materials I had to buy new was the timber framing and the osb sheets, concrete blocks and the sand. The plastic cladding came from ebay, major bargain it was, garage door company selling off a job lot. The roofing sheets were leftovers from a job at work that were going in the skip and I happened to be in the right place at the time to re-direct them into my trailer. Timber frame idea is easy to build yourself. Drew the whole thing in autocad and split it into sections to build. Once the floor level was complete I transferred the cad drawing for the wall from paper to full scale on the floor and cut and made all the sections there, same for the roof trusses.