Couple things. Cheap means heavy, generally, so if you're worried about going ultralight, "Cheap" isn't really going to be a thing.

Regarding packing, I typically just stuff my quilts into the bottom of my bag and forego even using a stuff sack. That way it can fill in all the nooks and crannies.

The gemini underquilt is a pretty decent cheap option if you're alright with DIY. To the best of my knowledge, though, it's only going to get you into MAYBE the 30s if you use the 3 layer winter mode. I'd say you're most likely to only get to 40 though.

If you're alright with DIY and don't mind weight, Almost any quilt or blanket can be made into a functional underquilt. At that point, it's just about adding suspension with some shockcord and beeners.

All that said, I don't know what your budget is, but winter UQ's are going to be inherantly a little more expensive. I know many users have experience with OneTigris. They have a winter UQ for under $100, but I don't personally have any experience with them, so might want to ask around.