The Dallas Morning News said Friday Ellis is expected to sign a deal worth $30 million.
Citing sources it did not name, ESPN said the deal would be worth at least $25 million, with the final amount depending on how much salary cap space Dallas has after it finishes signing players.
The Mavericks have already signed point guards Jose Calderon and Devin Harris during the off-season, and used their first-round draft pick last month to take Miami point guard Shane Larkin. Dallas also signed Israeli free agent point guard Gal Mekel.
Ellis, 27, averaged 19.2 points and 6.0 assists last season in 82 games with the Milwaukee Bucks.
He has averaged 19.4 points and 4.7 assisted per game in 516 career NBA games with Milwaukee and Golden State.