Courier has three paid pricing plans that discriminate based upon the number of notifications sent per month. This maps to how incumbent single-channel notification providers charge and I expect Courier to be able to charge prices comparable to a single-channel provider. The customer will be paying for both Courier and the underlying providers so it may result in Courier only being able to charge some fraction of a comparable single-channel provider.
There are at least two existing individual channels that support $100M+ ARR (and in some cases much, much more) and two more supporting $10M+ ARR in revenue for the leader(s) in that space: email (SendGrid), SMS (Twilio), mobile push (Amazon SNS, OneSignal, Urban Airship), browser push (Pusher). Achieving revenues in excess of $100M ARR is achievable.