If you go to their "packages" page[1] and select "premium", it gives you a table with "requests (per period)" and "month-to-month (price per period), and it is indeed "up to 100" for $99.00. For up to 500 tweets per request.

Seems pretty clear, though I can't fathom why they chose "periods" rather than "months". Crazy expensive.

[1]: https://developer.twitter.com/en/pricing/search-fullarchive

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edit: though the more I read it, the less confident I become. wtf is up with this copy.