I launched Pushover (https://pushover.net/) this week, which looks to be pretty much the same exact product as yours: an HTTP API to push messages to an iOS and Android client.
I built it over the past 4 weeks to replace my use of Notifo, which shut down last year. There are a few other apps already available, like Prowl, Boxcar, and NotifyMyAndroid, but none were cross-platform, so I built Pushover.
I am charging for my app as a way to pay for the server costs, as I'm not sure how else these types of apps can make money and stay around. Notifo had a lot of users but their apps were free and I don't know if they even had any paying content providers pushing large amounts of messages. How do you plan to make money with yours?