Absolutely gutted having just bought this for £40 from Play.com last week, its now £30.98 at Zavvi – even cheaper with a Student/New Customer code, bringing it to £27.88.
It seems to be a bit rare in the UK, and especially as each individual series seems to really hold its value (excluding the widely available first season).
But £15-20 a season otherwise.
Got into this show after 1 and 2 were on Amazon Video – and loved it.
Next best is still the £38.99 at Play.com and its £47-something at Amazon.