The ADuM4160 is expensive because it has five internal isolation channels. USB is a complex bus, as it's both bi-directional and differential. It's not easy or simple to isolate.
Also, the isolator needs to be fairly fast, USB-2.0 full-speed has a 12 Mbps signaling rate. Looking at a similar-speed 4-channel ADuM isolator, the costs are about the same.
Basically, there is no way to easily reduce the cost. Within the ADuM product line, the ADuM4160 is price-competitive with other devices of similar complexity.