There are weatherproof bivies that weigh as much, and sometimes cost more, than a tent. Then there are ultralight bivies that are actually sleeping bag covers for use with a light tarp. Never seen one (except maybe used) that cost less than $100 with maybe the exception of the silnylon/ripstop versions that are less breathable.

You are not going to get a weatherproof bivy that is ultralight.
_________________________
"In the beginner's mind there are many possibilities. In the expert's mind there are few." Shunryu Suzuki

http://hikeandbackpack.com