Gamora has a couple of looks in Guardians of the Galaxy. The costume I see cosplayed the most is the black leather ensemble, but the assassin also wears a yellow prison uniform in the film. If you want to wear a Gamora outfit but want to keep it simple and affordable, the prison outfit is the way to go. Replica Props Forum user JediSarah made her prison Gamora costume for about £60 or $93. She wants to make a more movie accurate version once she has the Blu-ray to examine for reference, but she said this was a good start.
She made the uniform/suit from a set of scrubs and used an old t-shirt for the darker yellow trim, and she wore a half cap wig and used Ben Nye makeup. The costume looks wonderful for the budget. You could pull together an even simpler version of this costume too by going with whatever shoes are in your closet rather than finding a match for Gamora’s and skipping the wig. Yellow scrubs are around $20. It won’t be perfect, but the costume will do in a pinch. That said, I can’t wait to see the improvements JediSarah makes to the costume after she studies the film.