You want Yamaha's YSTMS50 ($69.99 at Buy.com
) if you go 2.1. The best damn 2.1 set I've ever heard, and #3 in line overall when it comes to 2.1 speakers (#1 = Klipsch ProMedia 2.1, #2 = Altec Lansing 621).
Although you can get cheap Logitech 4.1 and 5.1 systems for under $100, no one in their right mind would recommend those (Z540 and Z640, respectively). You'll need to pay more than $100 if you want 4.1 or 5.1.