$0.0135 per message. That's almost like SMS, to be honest.
But extremely cheap compared to the markets they're targeting. SMS is expensive in countries outside of the US, and that's why WhatsApp is so widely used.
Is it cheaper than the same functionality in WhatsApp's business API?
Unknown, it's not currently public pricing afaict. Probably behind an NDA or variable,
$0.0135 per message is for messages to US numbers only. E.g. price to Belgium is a whooping $0.1010 per message.
Almost like Twilio's SMS price, but in the real world SMS in the US is ~$0.0015 (outbound) and free for inbound and that's without trying very hard, can get even lower. Granted it doesn't come with all the ready-made tooling and bells and whistles, which evidently has a high price tag attached if doing via Twilio.