Looking for an ultralight double sleeping bag system for alpine bivy's. Nunatak Dual Alpinist looks good except for the price. Any other options out there?

You could make your own quilt (or even a full bag if you wanted), for cheaper than a manufactured one. Partially depends on your sewing skills and patience, but it really wasn't too hard.

Mine weren't doubles, but I made a 32 degree quilt and a 40 degree quilt for about $55 each if I recall correctly. I'm sure making them into a bag wouldn't be too terribly difficult.