Or, it might turn out to be zero. The article left it unclear whether they (the owners of the platforms) are making or losing money by doing this.
But there are two other concerns that would have to be addressed: Plugging the oil well, and navigational hazards. Neither seems to me to be insurmountable. Plugging the hole and leaving the platform there is easier than plugging the hole and removing the platform. And the navigational hazard has been there for decades. Leaving it changes nothing; an existing risk merely continues.