Cash Back Event

What kind of NX purchase would they even be thinking about, future speculative or not, that would cost even 90k NX? o.O

That's cumulative value during the period.
It's not cash-back per item.

They're rolling the entire sum of your NX purchases from the 16th to 22nd together.

If you spent <16K but more than 10k you'll get 10% credited back.

If you spend more than 16k you'll get 15% of that total back to you, up to a limit of 15$ for reaching 100$ or more. If you spend 300$ on various things during that period you're only going to earn 15$ back, not 45$. Unless of course you happen to spend 100$ on three separate accounts (not characters) then you'd get 45$ and if you intended to spend the 300 anyway that'd be a sneaky way to get the most from them.