If I can comfortably go to 0 for $50, I will be very happy indeed!
First thing I'll say is that you may experience drafts in a quilt when you move around. I've only taken mine into the high teens, but my frequent movements was starting to make the drafts annoying. I bet it would have been very irritating if it was 0° F. That said...
700 fp down from Feathertex. You have to order a minimum of 2 pounds, so that's $78. Let's say you use 7 yards of 1.1 ripstop nylon from Backwoods Daydreamer for $21, but I think you would only need 6 yards. You may also need to buy thread, a little grosgrain, some shockcord and some hardware, depending on the design you choose to make...so I can't really come up with an estimate for that stuff. For the down and fabric you're at $99.
With your dimensions at 54x42x78x4, you'd have a very wide quilt <2 lb for a 70" tall person. My estimate is that it would be good down to -10°F. You would have a little over 6 ounces of down left. Hmm, my estimate was bad. I think that was based off of 3" of loft and selling the spare down. I believe 3" of loft would be good down to 0°F. If you're slim and sleep on your back, you could go with a much narrower quilt that weighs less and leaves you with more leftover down.
One thing I like about this type of quilt is that you can build it very warm, but still open it up completely like a blanket to stay comfortable on warmer nights.