So far, I've unlocked two costumes by playing Arcade on the first difficulty. Third run didn't yield anything. Then I played a round of Survival on the first difficulty and that unlocked another costume.
These are all for Kasumi. If I remember right I have her blue default, white default, black default, and flower dress. I'm missing the slick new black costume that looks Ninja Gaiden-ish, and her blue kimono.
I'm not sure if difficulty matters or not. Hope not. Because I suck. lol
Dividing my time between this and Borderlands 2 so I'm not sure when I'll get around to trying for more. Does Time Attack yield anything? I usually stray away from time-based modes.