Earlier posts mention estimated compliance costs for the new system.
Methinks the astute beancounter will be looking for savings, not costs. For example, there should be a measurable savings in hull and liability insurance cost, once the inerting system is mature enough to save an airplane every few years.
It's usually too easy to point to costs, without considering the flipside.
In the scheme of things I don't believe the insurrance carrieres care a twit about reducing premiums for so little reduction in risk to their bottom line. They have a tendancy to look at the worldwide overall risk to themselves for all carriers and all transport types.
After that you may argue about the saving of one B747 in some many years (is it 1 out of 20 yrs? and maybe argue for 10% of this as kickback insuree credit spread out over all operators of the type.
extremely insignificant compared to the costs of the retrofit.