Seasons One through Four are on sale at Amazon between 36% and 60% off. I've never seen the series but I've only heard good things about it. Totaled up they come to $123.95 which is just $24.79 a piece. Not bad for television series' on DVD.
Season 1
Season 2.0
Season 2.5
Season 3
Season 4
Season 1
Season 2.0
Season 2.5
Season 3
Season 4