The 22-year-old striker had a sensational impact at San Siro after joining from Manchester City for €23.4 million in January, scoring 12 goals in 13 appearances for the Rossoneri as they clinched third in Serie A.
Galliani believes the mercurial forward will prove a significant bargain for the club given the relatively small transfer fee is still being paid in gradual installments.
"He is a top star, even if he was often a reserve at Inter and Manchester City," the 68-year-old told reporters.
"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 instalments."
Balotelli's strike partner Stephan El Shaarawy has been linked with a move away from San Siro in recent weeks after suffering a dip in form since his compatriot's January arrival, but Galliani has stressed the 20-year-old wants to stay at the club - despite receiving a substantial offer from an unnamed side.
"Stephan received an important offer with super wages for him," he confirmed.
"But the lad asked to stay and we were happy to keep him."
Galliani, who has been at Milan since 1986, was also quick to praise the consistency of his side and expressed his faith in their ability to regularly finish in the top places in Serie A - unlike some of their rivals.
"Milan never go below third place, as over the last five years we have always been on the Serie A podium," he continued.
"When our season went really badly, we finished third. For other clubs, such as Inter and Juve, when they had a bad season they didn't even finish on the podium.
"Juventus are still in pole position for the Scudetto, because they won it last year. However, Milan are right up there, so at least we'll be in the top three."