From the NWPath-Documentation "isExpensive - A Boolean indicating whether the path uses an interface that is considered expensive, such as Cellular or a Personal Hotspot."
How is "isExpensive" meant?
In regard to money, consuming system-performance?
isExpensive property from NWPath is not determined by Apple. It's just a computed property that rely on network interface metadata from system - kernel networking. You may want to take a look at this.
In regard to money, consuming system-performance?
IMO, it could be both of these things. Why?
Network framework inspects the interfaces that made up the current path, are any type of cellular such as LTE, 5G, etc. The NWPath is considered expensive, because it will cost the user money.cellular, the system often consumes more battery. Because it use more energy to maintain the connections (depend on signal strenth, distance, etc), compared to Wifi.