Is it always advised that apps are built for the specific platforms they are on for the sake of quality and usability. If you are on a budget and want to have your app on all platforms you are able to use Phonegap, a non native code. Whether this option is suitable for you depends on the nature of your app and what you want to achieve overall.
While building a BB10 mobile app may not be practical for start ups, Android apps can be ported across to BB10 based devices in a few click, so why not add your shiny new app to another app eco-system, for a low cost, as Blackberry app store is far less fierce, having your app on their store could boost your brands aware via the niche mobile platform.
Being the largest mobile market, Android has grown at an incredible rate, if are looking to target a large number of users, building a mobile app on Android is the choice for you. Your new shiny app, once completed can be on Google play store on the same day it is submitted.
iOS apps are where it all started, while it may not have as many users as Android, successful IOS apps can be hugely rewarding if executed correctly. IOS users tend to be be the biggest app spenders, so a carefully crafted mobile app, on the iOS app store can lead to great steady stream of additional income.