For matrixing on-site in an open concert environment, you really want either closed (gloco has recommended a couple) or in-ear phones. I highly recommend the in-ear Etymotic ER-6, and they're only slightly above your $100 budget. WELL worth it, IMO. They do a great[/i job attenuating external sounds, can be driven easily by most on-board headphone amps, and sound mighty fine.