Homemade is even cheaper. A bivy is a simple first project. The BPL pattern's not hard to reverse engineer. I have a homemade bivy and a bmw nano, and haven't noticed any performance difference. In fact, I prefer the homemade because (1) it's the work of my own hands (however poor) and (2) I chose the colors.
The cost of a homemade bivy is about $40, or less if you use seconds fabric.
(My gear choices are perfectly rational, as you can see.)