The next best prices for the 3 volumes individually:
Vol .1: £5.09 (released 07/05/07)
Vol. 2: £13.43 (released 16/07/07)
Vol. 3: £13.43 (released 17/09/07)
Basically, Play are offering these DVD's together for 3p under half of what you'd pay if you bought them separately. Not bad.
Vol. 4 doesn't come out until next month.