I do have a -20 F down mummy and size L Thermarest X-Therm for winter so yeah, I ain’t “poor”.
But if you have a true 20 F. mummy or really good, non-drafty 20 F. quilt you can extend its range with the following:
1.)  light to medium insulated jacket  2.) down or synthetic insulated pants.  3.)  stand alone down hood or really warm balaclava. 4. fleece gloves (Yes, ‘sleeping gloves” are part of a winter sleep system).
Plus the “foundation”-> 5.) double layer Z-Rest or RidgeRest OR a foam filled mattress with one of the above closed cell foam mats beneath. And maybe an R 3.5 insulated air mattress on a closed cell mat.
With these add-ons, most of which can do daytime duty as well, you can very likely take that 20 F. sleep system to at least 0 F. In many areas of the US that should do for a winter sleep system.
