Buying at a comparable price for that many kids you're probably going to end up with sub-quality pieces.  I would recommend looking at alexanders costumes (either alexanders dot come or alexanders costumes dot com, can't remember).  Call them to set up a wholesale account as a theatre if possible.  You can get harem vests for around $6, harem pants about the same.  Their shiny pirate pants will last for a full run, but maybe not longer than that.  The cotton ones are FANTASTIC and will last a long time (washable!).  If you have a creative costume person you could really end up being able to costume the show on a great budget by purchasing some pieces you can find affordably and renting by the piece the things you can't find.