If you're open to a quilt, it can be a bit cheaper.
Seems the standard for 0 degree bags is three inches of loft, so assuming your quilt would be 50 inches wide, you'd need (70 in)*(50 in)*(3 in)/(850 in^3/oz) < 13 oz of 850FP down; if you want to overstuff by say 20% you'd need about 2 oz more, so 15 oz total. $30 of savings.
Additionally, taffeta and 20D are nice, but I'm a cheapo and am just fine with plain old 30d ripstop seconds, which can be had for a mere $3.20/yd, saving you about $40.
So there's a $110 zero degree myog *quilt*; obviously there's some decreased functionality from your proposed design and materials.
EDIT: For a true bag, you could probably get away with the equivalent of a 53-54" rectangular quilt (assuming you're going hoodless), which would require 16 oz of 850FP down with 3 in of loft and 20% overstuff. Also, don't forget to account for the material for your baffle walls! Noseeum would be cheaper than the 20d or tafetta.