NBA executive vice president Kiki VanDeWeghe announced the fine on Saturday. A league statement said Atkinson was fined for "verbally abusing game officials and failing to leave the court in a timely manner."
Atkinson was ejected with 1:09 left in the game after Brooklyn, trailing by five, did not receive a foul call on one end of the floor, and the Nets' Rodions Kurucs fouled Darren Collison on a fast break on the other end.
Atkinson left the bench area to confront referee Tony Brothers and was immediately ejected.
"We've just got to be mentally stronger, and I've got to be mentally stronger, fighting through those situations," Atkinson said of his ejection. "Just frustration on my part. I have to coach myself. I don't love that scene out there. That's not what I'm there for. I'm not here to make a show. So I apologize for that. That's not who I am. But it happens in this league sometimes with the emotion and intensity of the game."
Nets forward Jared Dudley said he didn't have a problem with Atkinson's ejection.
"I think it was justified. You want your coach standing up for your players," Dudley said. "If he's out there screaming, you know there's a reason why. You want your coach to have your back, and he took those two techs. Now you know you have someone in your corner fighting for you."