Twitter's enterprise pricing costs so much it dare not disclose the figure publicly; you have to ask the company to reveal the numbers.
And in an arduous journey through their corporate layers ask I did. Supporting Twitter in its entirety would raise the costs of Syften so much you'd probably laugh. Nevertheless I have found a way to receive about 12% of all English tweets matching the keyword "https" while keeping the subscription costs of Syften reasonable. If a Tweet gets retweeted 10 times there is a (very roughly) 1-(.88^10)=72% chance we'll get it.
PS. On 14 May Twitter added a new Filtered Stream to it's "Incubating" category on its public Platform Roadmap. The description is inviting and I hope to improve my Twitter coverage once it's released. Please contact me to be notified of any changes to how Tweets are filtered.
Advanced sifters will find these details useful: