Unfortunately, with the enterprise signing bug going away in iOS 8.1, emulators like GBA4iOS, the one that seemingly popularized the whole trend, will no longer be installable by anyone without jailbreaking or having a developer account. However, the date trick to install the app still works for now, and so Riley Testut and the GBA4iOS team are putting out GBA4iOS 2.1. The source for the app should be available soon in beta for testers with developer accounts to install it themselves. Multiplayer is the big feature of 2.1, but isn’t going to be part of the initial release as it’s still in the works. 2.1 will add iPhone 6/Plus support, the ability to set a homepage for the web browser, and selectable color pallettes for Game Boy games. Testut says “Under normal circumstances, I’d much rather hold off on the release of the update until I had completely polished it, but right now our priority is to get the update out before iOS 8.1 drops for everyone."
So, in the long-term, the plan is for the app to be available on Cydia for jailbreak users, and available in open-source form for anyone with a developer account to compile and distribute to their registered devices. As such, that $99/year might be worth it if you want emulators on your iOS device, and you find Android, a land where emulators run wild and free, too distasteful.
