The figure of £5,000 for a Spitfire was a nominal target for the organisers of "Spitfire Funds" to raise. The MAP agreed prices with manufacturers for each batch of aircraft, although these were for the bare airframe and didn't include engines, guns, radios etc. That might explain the disparity between the figures of £22,000 and £42,000 for a Lancaster - unless one is a misprit! I found a note among Harris's papers at Hendon, giving a breakdown of costs (such as £2500 for an engine, £45 for a Browning and £355 for an 8000lb bomb) but not a cost for the bare airframe or complete aircraft.