The Italian international striker joined the club from Manchester City for £20m in January and went on to score 12 goals in 13 games.
The 22-year-old's form helped the Rossoneri secure a third-placed finish in the Serie A table.
"He is a top star, even if he was often a reserve at Inter [Milan] and Manchester City," Goal.com quotes Galliani as saying.
"When he arrived at Milan, I told him to continue being a star, but a starting one. I consider him to be a bargain, as he scored 12 goals in 13 games and we are paying for him in monthly installments."
Balotelli spent three years at City before joining Milan.