A few of my favorite things:
Arm warmers,
a really lightweight windbreaker vest that scrunches up into a tiny ball (like my PI Optik),
Terry Bella shorts and knickers (probably no longer under $100 though ),
Smartwool socks,
a Buff,
my Mountain Feedbag (http://www.epicrideresearch.com/product-information.php), and
those wicking beanies or doo-rags- they keep the sweat off my face.