The cost of the diversion was not even remotely close to $90k, where are you getting this from? Much more likely closer to the $9k figure. Per the article linked by the OP:

"Federal prosecutors in Tulsa say 29-year-old Bolutife Olorunda of Vancouver, Washington, pleaded guilty Tuesday and will pay Delta Airlines $9,118 for the cost of diverting the aircraft."

The article states the cost of the diversion was $9,118.

In this article:

https://thepointsguy.com/2017/08/haw...ssenger-fined/

The Hawaiin Airlines A330 (wide body) had to turn around in flight and return to Honolulu, fine was $97,181. "The fine is meant to reimburse Hawaiian for the losses incurred to turn the plane back. Costs included fuel, maintenance, ground crew, a new flight crew and re-accommodating passengers on other flights." So, a wide-body that had to turn-around after being in-flight for 2+ hours (thus 4+ hours of wasted fuel) and had to pay a new crew and re-accommodate passengers on other aircraft had a total cost of $97,181 (this doesn't include meal vouchers the airline had to give once the passengers were taken off the aircraft). Yet, you think a narrow-body aircraft that used a relatively small amount of additional fuel, since it stopped en-route, where it then continued on without changing crews or disembarking passengers, would cost about the same? No way. During landing the fuel consumption is minimal as the engines are running at much lower power levels during the entire landing process.