The Avid BB5 or BB7 disc brake will come with the rotor, caliper, and mounting bracket. You will need the cable and housing and a disc compatible front wheel.

The BB5 brake will help keep you closer to your budget at $31:
http://www.speedgoat.com/product.asp...at=26&brand=16

New cable and housing for $1.99:
http://www.niagaracycle.com/product_...oducts_id=2238

And a front wheel for $45:
http://www.niagaracycle.com/product_...ducts_id=20793

Under $100 with shipping. I'd recommend having that wheel retensioned at a compentent LBS though.