Nike being the big player in this field had to come up with strong statement against it's competitors. Yes, it is ridiculously huge amount but they had to do it with level of competition in today's modern business. Celebrity are looked upon by ordinary people, they set trend and other people follow them.
Businesses spend a lot of money each year on celebrity endorsements to advertise their products. This method of advertisement has shown proof that it pays off big time. At the end of the day, when a company sends a few million dollars on a big name endorsement, they have usually done enough research to figure out whether that investment will pay off. It almost always does.
I am not supportive of the huge celebrity endorsements, such as the Nike Durant Deal. The sport has to be a game, enjoyed by thousands of millions of fans and aspire youngsters to move, run and play. Instead it has turned out into a rich people show-off and a treadmill for rivalry business counterparts. Such amounts are better to be spent for charity, to fight poverty or illness.
No, it is not worth it to spend that much money on celebrity endorsements. I don't think having a celebrity will necessary increase your revenue or product sales by all that much. Spending $300 million just to have a celebrity endorse your brand is a tremendous risk and may turn out to be a complete waste of money if no one finds the endorsements appealing.