In FM time my best signing was in FM2007 Freddy Adu in mid-season for about 1M.By the end of the season he was lague leader in assists.And the next season too!!!
But the teams i ever assembled was in CM4 with Inter.I bought Ronaldinho from PSG,Diego and Robinho from Santos,Kaka (from Sao Paolo i think) and many others of smaller calibre.Don't ask the exact money,coz i don't remember.But afterwards i was in debt...
However the best two signings i ever made were for my CM01/02 game with Leeds.In the 3rd season i signed Roman Vasilyuk for 10M and he scored in 52-53 games 55+ goals!!!And next season i bought Zlatan Ibrahimovic from Ajax for 8-9M and he scored about 40goals and then led (my) Sweden to the World Cup!!!
That were good times...