sleeping bags are definitely a you get what you pay for type thing. There are a few down bags from China on Amazon at amazingly low prices, but they seem to be fitted for leaner builds and higher minimum temperatures.

It doesn't meet your budget, but my original thought is an Enlightened Equipment Prodigy quilt 6' wide: http://www.enlightenedequipment.com/prodigy/

$175 and 27-28 oz for a 30 degF bag. $185 and ~34 oz for a 20 degF bag.

*edit* I should add, I am pretty close to your build and a 6' wide EE quilt works great for me.