That is unfortunately with everybody else you cannot sign outside the initial starting month of the game, as either the teams will quickly realize their talent/worth or they transfer/sign mega-contracts. If you are lucky you might find them on transfer list because they end up disliking their boss, want to move to a higher rep club, or their contract nears its end.
As for Damiao, I feel he is worth every penny you spend. He's got the top-class physical stats to thrive in the EPL, with the mental and technical stats to compliment and make him a dominate striker!