Hey Joss, Danny here from StatusPage. We initially charged per notification, but it felt a little nickle and dimey and decided to switch to a limit of subscribers with unlimited notifications sent. Some customers didn't use notifications just because they didn't like the thought of a variable cost associated with the account.
We recently made this change, so feedback is appreciated. I'll send you an email to discuss further.
Plus, why are you limiting subscribers so much? 500 max for the startup plan?