Also being short on funds, I opted for the Equinox ultralight bivy. The Equinox was evaluated along with about a dozen others in the Backpackinglight magazine, vol.7, and was one of three or four that earned a top average score. It weighs in at 6.3 oz on my scale (BPL said 7), has a WP silnylon bottom and WR/B top, but no bug netting. I overcome this by wearing a headnet but have plans of sewing on a piece of netting. This is just the ticket for use under a tarp, and I have even used it as a stand alone piece of equipment in the open, weather permitting.
I started out with a much more expensive Integral Designs bivy that had a proprietary coating and was supposed to be WP/B; however, I found the level of condensation unacceptable (i.e., it did not breathe) and water could pass through the fabric from the bottom due to hydrostatic pressure.
So in my book the Equinox was a terrific find and functions well, except for the bug protection, which is easily overcome. There are a lot of good choices out there, but they'll cost you a lot more, too. Check it out at: equinoxltd.com. Happy Trails!