Ask HN: Web TTS API recommendations?

Hey folks, it looks like we're outgrowing our free Google web TTS API limits and we need to change providers.

Ideally it'd be a service with free or extremely affordable rates, a very clear SLA, a reasonable voice quality and a pretty brain-dead HTTP API. There are some interesting options out there: Microsoft Translate, Amazon's IVONA, tts-api.com and many many others, seems like most of them either fairly pricey or with non-obvious SLAs.

Would be very curious to know if any of these worked particularly well for you and if there are caveats to look out for?