You can redo maps for Extend coins. You also get a very tiny amount for certain dialog during the map. Example: Having Amuro attack Char and initiating their conversation will net you 5 coins the first time. If you redo the map and have them talk again it is 1 coin. (This is aside from the map completion bonus coins)
It's not that I'm low on them or anything it's that I just wanted to make sure the game didn't depend on you spending real life money to get them. That's one reason I waited to buy the game before now.