This is how I'd dot it (one of the ways I can think of):
I considered that the IDs of Mobile Devices and Desktop Devices are unique, therefore you can create a Mapping table containing all the Devices (both mobile and desktop) and apply the mapping onto the final table. After that, you can get the corresponding Cost from the Mapping table by using the ApplyMap function in the final table where you have the Device column.
1. Load all the Device data into the same Mapping table:
"Device Mobile" as Device,
"Device Desktop" as Device,
// This would result in one table.
2. Load the facts and apply the map from the previous table to get the Cost:
APPLYMAP('MAP_Devices', Device) as ItemCost, // just in case you want to keep the individual cost of the device
APPLYMAP('MAP_Devices', Device) * ItemCount as TotalCost
Hope it helps.