No, Ariana Grande did not receive fifteen times as much money for Wicked as Cynthia Erivo did. The studio denies the rumor of pay disparity
According to a studio representative, Ariana Grande and Cynthia Erivo «received equal pay for their work on ‘Wicked,'»

Following online allegations of a significant salary disparity, the studio stated that Ariana Grande and Cynthia Erivo earned the same paychecks for Wicked.
«Any claims of a wage gap between Cynthia and Ariana are untrue and based only on online rumors. In a statement released by The Hollywood Reporter on November 26, a Universal representative stated that the women were paid equally for their work on Wicked.

Following now-disproved online rumors that Grande received $15 million for playing Glinda and Erivo received $1 million for playing Elphaba, the studio released its statement. The precise sums given to the actors were not disclosed by the studio.
Both Erivo, a twice-Oscar-nominated performer with a Tony Award, and Grande, who has a built-in fan base from her music career and an impressive 376 million Instagram followers, have gotten a ton of praise for their roles in the movie.

Director Jon M. Chu contemplated using «no-namers» in place of Grande and Erivo as the main characters in the Wizard of Oz prequel movies.»I wanted to have a very clear slate coming into Wicked,» Chu, who is 45 years old, stated. «We can find two persons because the property is large enough on its own. ‘We’re going to find no-namers,’ I thought.
«But then we got calls from all these great actresses who wanted to audition and we saw everybody, and they were all really great,» he stated.