What is your priority? you get to pick two of three characteristics - light, warm, cheap.

For the lightest, most long lived, and most compressible bag for the rating you want - 800 or 900 fp down is the way to go. Unless you are hiking for weeks in 90-100% humidity.

For the compromise, a lower quality down bag will be heavier and pack not quite so small, but will be cheaper. There's a Campmor 0 degree that's probably 10-20 degrees warm - the 20 degree Campmor down bag is probably a 30 degree bag in practice. The Kelty Cosmic you mention is similar. 600 fp down has more feathers than the more expensive bags.

Synthetics will be bulkier for the rating, and wear out in a few years of frequent use. There are places and reasons to have a synthetic bag - folks often prefer them for winter.

I am ultra-cheap, so rather than replacing the bag every few years, I sprang for a 800 fp down quilt. It weighs less than a bag and has no chance of a zipper jam, and has been great down to the expected mid-20F range. For $250 it was a steal. I anticipate years of use out of it.