I concur with everything Phat said. Your problem is underside insulation. $400-500 WM bag isn't going to keep you any warmer. The reason the WM bag weighs less than your current bag is two-fold, 1) higher loft down, and 2) WM doesn't put much down on the backside of the bag... because it doesn't insulate you anyway.

Since your backside doesn't need to breath, vapor barrier type insulations work very well like the blue foam pad Phat recommended. If you want to get a quilt for under $200 for backside insulation, I would check out EnLightened Equipment.