The question would be, do you want negative 20 to be the comfort rating or extreme?
If it's comfort you want, then you really need to be looking at -40 degree bags or so.
Feathered Friends, Western Mountaineering, these would be the premium no BS will work option, but are a grand or so.
I was going to go with the Western Mountaineering Cypress expedition weight, this is -30, then have them overfill.
I have decided I hate being in a burrito enough that I've changed that, I'm going to go with an enlightened equipment -10 quilt with another, likely 10 degree quilt. They note that should be good to about -70, but that's extreme rating, given quilt... I think it'll get me to -30 ok.
One of their 0 degree and a 30 degree they note would be about -40, that's extreme rating again, but would give you two separate quilts that should work from that -20 to about as warm as you'll ever hunt given you can open that 30 degree up fully into a quilt and not worry about cooking inside a burrito.
None of my options likely fit your $750, in fact, for me, all my options will be about $1300-$1500.
But that kind of cold, it's tough to gamble on unknown China or the like quality, if you do, just be aware of that and have good backup, like extreme cold down garments or something.