In recent times, the best bargain I can think of is Dortmund buying Shinji Kagawa for 350K euros.
Yep, surprising aswell because he was already quite established in the Japanese league.
Henrik Larsson - 650k Celtic
David Platt - 200k Aston Villa
Kolo Toure - 150k Arsenal
Dixie Dean - 3k Everton
Liverpool-related bargains - these have been mentioned but i've added the inflated 2012 figure in brackets aswell.
Ray Clemence - 18k from Scunthorpe in 1967 - (270k)
Bruce Grobbelaar - 250k from Vancouver Whitecaps in 1981 - (880k)
Phil Neal - 66k from Northampton Town in 1974 - (650k)
Markus Babbel - Free from Bayern Munich in 2000 - (Free)
Alan Hansen - 110k from Partick Thistle in 1977 - (650k)
Sami Hyypia - 2.6m from Willem II in 1999 - (3.6m)
Steve Nicol - 300k from Ayr United in 1981 - (1.1m)
Steve Heighway - Free from Skelmersdale United - (Free)
Emlyn Hughes - 65k from Blackpool in 1967 - (980k)
Graeme Souness - 350k from Middlesbrough in 1978 - (1.8m)
Jan Molby - 225k from Ajax in 1984 - (630k)
Gary McAllister - from Coventry City in 2000 - (Free)
John Barnes - 900k from Watford in 1987 (2.2m)
Peter Beardsley - 1.9m from Newcastle in 1987 (4.6m)
Ian Rush - 300k from Chester City in 1980 - (1.3m)
Kevin Keegan - 35k from Scunthorpe United in 1971 - (460k)
Kenny Dalglish - 440k from Celtic in 1977 - (2.6m)
John Toshack - 110k from Cardiff in 1970 - (1.5m)
Ian St. John - 37.5k from Motherwell in 1961 - (710k)