As usual, sorry if this is posted elsewhere.
Morrisons are currently selling each Coast boxset for just £9.99. This is valid for Series 1,2 and 3. I don't think there is a 4th yet!
Its instore only, and dependant on stock (some only have x series, others have them all).
Amazon are more expensive for every series, not checked elsewhere, but I don't think anything will be that close.