I don't doubt that you'll be able to cover it on the large whole, since only a fraction of users would parse JSON bodies that large, however, those heavy users

I don't doubt that you'll be able to cover it on the large whole, since only a fraction of users would parse JSON bodies that large, however, those heavy users will probably use a large portion of the resources. It's hard to weigh conversion numbers vs heavy usage, a low price like this will certainly help adopt new users and in large numbers. It might even make it worth tolerating the (few) heavy users. Or you put it in cloudflare workers, which can parse 1.5MB of JSON within 50ms CPU-time and call it a day (10M requests/5$).