If you have a mutli-region DVD player, you might want to check out Amazon.co.uk before ordering these, I've noticed the US releases of British shows are insanely expensive. Even at these prices, Amazon.co.uk is a bit cheaper, especially if you buy multiple sets to divide shipping. (Seasons 1 and 2 are roughly $25 each, the 4 season box is about $100 total, etc; shipping adds around $5 + $2 per DVD) And that's just checking today when they're not on any special sale in the UK.
(Granted I wouldn't suggest anyone just randomly do this unless you know your DVD player can play Region 2 PAL discs but if you do, this deal suddenly becomes a fairly average price compared to Amazon UK's every day price)