A cheaper alternative is to get some Avid V-brakes,
http://www.jensonusa.com/store/produ...ull+Brake.aspx

Should give you better braking than the no-name ones that came stock. While you're buying them, get some spare kool-stop pads for when the stock wear out.

Discs are definitely better, but getting it under $100 is difficult. don't try to get it cheaper by buying a lesser known brand.