The word translated as "hardly" and "hard" has the primary sense of "with difficult," not that it isn't often done. However, it also has the sense of "peevishly," unpleasantly," and "discontentedly," which describes as state of mind. This is a good example of why "the kingdom of heaven" might be best expressed as a state of mind rather than a place. It perhaps tells us more about the "kingdom of heaven" than it does "the rich."
This phrase "in the realm of the skies" takes two different forms in Greek. The form here with the preposition eis is only used three times by Jesus. See this article about this phrase and its uses. This phrase is not as common as you might think, used by Jesus only four times in this form.
Christ's Words in Matthew as a Guide to 40 Days of Prayer.