I mean, the AI partner is probably getting paid, so I can see DuoLingo needing to increase their rates if you use that service.

I'm not sure exactly what method they're using, but inference costs for speech to speech models are pretty significant so it does make some sense to charge more. The LLM-based text explanations for translation problems are also in the super premium tier though, and those can't be that expensive, so maybe it's just an attempt at market segmentation.

The LLM based explanations actually explain what is wrong with the user's specific answer, not a generic explanation. They are of the form:

"You entered X, because of Y grammar rule you should instead enter Z".

So they aren't something that can be pre-generated for all possible inputs. Some of these questions are free-form input.

That's fair. I've just been screenshotting the page and asking Gemini what I did wrong for free but I guess Google has deeper pockets than Duolingo so they can eat the cost of that for longer.