First thing look in your closet, see what might create the look you are going for. Peasant Skirts, little white shirts, with elastic necks or peasant skirts are all great for this look.
This shirt is from http://redzippervintage.etsy.com/ SOLD!!!!!
This shirt is a great Pirate Wench look. You could pair this simple shirt with a wide black belt, a fabric sash or a modified Vest. Off the shoulder or on this has a great feminine look, that works for this look.
Do you have an old vest in your closet? Like this one? From
http://Trashique.etsy.com/ for $5
Cut the vest just below the bust line, hem, add trim and get some skull buttons or hooks. To create an inexpensive Half Bodice look, that is super simple to create!
Peasant skirts that are full and blousey are a great way to go, and as you saw from my costume example, if you cut the bottom into rough jagged edges it has a great pirate feel. Do you want to re-wear your skirt? Then simply tuck up the edges into your waist line or a belt and still look great when you wear the skirt to work the next day!
Accessories make a costume in my mind, they are what adds depth and interest to the costume.
You can wear simple leather boots for your feet
A fabric bandana or a cloth sash made from a simple scrap of material work to create simple pirate hair.
Lastly Jewelry, go to you jewelry box, take out all your pearls, anything silver or gold, anything big with "jewels" and put it on!! More is better in this case!
If you don't have all the pieces above then then try Etsy and Thrift Stores they are your best friend!
Using the two pieces above, a skirt from your closet, and your own accessories you could have a complete costume for under $20.