Interesting idea. I've spent days looking for patterns that match a specific character or piece of a costume.
honestly, if you just did parts in generic sizes you'd have a pretty good market I think.
I'd buy them.
If you can produce copies of them, charge like $10-20, if you're doing custom I'd say $30-50 maybe?
Cosplays in Progress:
Haruno Sakura (Shippuden)
Theodora (Oz the Great and Powerful)
C. Viper (Street Fighter)
Twilight Princess/BRAWL Zelda (Green BRAWL ver)
Tharja (Fire Emblem Awakening)