What is a good thing to have for a small home studio?

I'm looking to expand my small home studio. I have a bunch of decent speakers (not super high-end, not crap either), an old home theater receiver, an old BSR-EQ3000 equalizer, a CAD GXL-2400 microphone, ART Dual Pre USB interface, and cables/stands. Anything else I should buy? Something around $100.