Cash Back Event

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.

Considering there are two periods of "cash back" as well, if you stagger your charges and spend, say, $100 on your main account during each period, you'd get back a total of $30 after the $200 spent. Or, as Eos said, spending say $600 on three accounts split between two periods ($100/account/period), you'd get $90 back.