I'd say you have to make the game first, otherwise the question makes no sense, especially taking in account dynamics of mobile games market. Games could be very different, and the answer depends on it.
In general, Apple doesn't allow emulators. Only someone like SEGA can make one and get it in the app store. If I were going to write mobile applications, it would be for something like Android, not the iPhone.
Ports usually pay very little - porting a major title from the PC to OSX, for example, usually only pays a few thousand. The older the game, the less a port is worth to a dev. Get old enough and you probably won't get permission to port it. Old games that went open source won't pay because you can't use the game data files for commercial purposes.
Unless you have an in with a major distributor, most devs do ports as a hobby as there is almost no money in it otherwise.
I had no idea.