How does App Store "complete my bundle" work when I've spent more than the bundle cost?
Since you’ve already paid more for one app than the total cost of the bundle, you get the rest of the bundle for free. This is why it says GET without a price.

If you had to pay anything, it would show you a price instead. For instance, in my case I see $3.99 where you see GET.

iTunes albums work the same way. Every time you purchase a track on an album, the cost to complete the album goes down, as reflected on the Purchase button. If the cumulative cost you’ve paid for the tracks on an album exceeds the cost of the album as a whole, the button changes to GET and you can download the remaining tracks “for free.”