Curious, when do you use credits versus cash?

I bought the two credits a month for a year deal on prime day. I’ve noticed since then many of my titles on my wish list are under the classic $7.49 whispersync mark. This post isn’t to talk about speculation as to why on that, but my curiosity is piqued.

I spend ballpark $7-9 per credit. I have lots of them and want to use them. I also get another book a month with Amazon music. At what dollar figure do you say “okay this is worth a credit” versus paying for it outright?

Personally I’m going with under $4, maaayyyybe $5, but because I have so many credits I don’t mind taking a couple dollar “loss” in value. Thoughts?