If you get an annual Audible 'subscription' you're looking at something like £7 per audiobook. (Oddly they don't advertise that this package exists, but it does.) Even less than that if you play the game and go for Audible's promotions.
> Let me buy books on the app
I agree this is an odd omission. Could it be something to do with Apple's rules about handing over their cut? They gave Spotify a hard time a while ago - https://www.theverge.com/2016/7/1/12082398/apple-music-spoti...
Edit: apparently the Android app has no such restriction, so I suspect I may be right. https://news.ycombinator.com/item?id=16324731
Edit2: Yup - https://www.quora.com/Why-doesnt-Audible-allow-you-to-purcha...