A Football Association panel imposed the fine after the incident in the Second City derby in March.
Thug Paul Mitchell, of Rubery, was jailed for 14 weeks for the attack, which took place in the ninth minute of the derby clash last season.
Birmingham City were found by the FA "to have found to have failed to ensure that spectators, and all persons purporting to be its supporters or followers, conducted themselves in an orderly fashion and/or refrained from improper and/or violent behaviour and/or refrained from encroaching onto the pitch area."
Grealish recovered from the attack to go on to score the only goal of the game.
The victory was part of a winning run which took Villa to the Championship play-off final and eventually secure promotion to the Premier League.