I think it is actually canon, not headcanon, that Parker likes cash best. (what follows is headcanon) She loves her expensive gems and so on, but cash money is real, it's there, and she loves the way it smells, the way it feels. She grew up on the streets, more or less, and I think money represents a sort of stability for her that she never had growing up.

Sarah doesn't take sedatives - despite her suggesting them to York, she prefers not to rely on them either. She's had experience sleeping in uncomfortable situations - on the road, on the run, in sketchy motel rooms, dodging police and FBI and Terminators. She isn't getting the best sleep of her life on Sacrosanct, but she's able to put aside all those issues and get the sleep she needs, for the most part.