Definition of cost in online advertising
When calculating CPM, or any equivalent metrics, cost is defined as the total media cost for the period of time and/or the volume you buy.
If you are calculating the total CPM for one specific week, you use the total media cost for that week.
It's up to you if you want to include other costs related to the advertising, for example a fee to a media agency or a system supplier. But keep in mind that you should be consistent; if you include other costs when calculating CPM for last year, you should include the same costs this year to get a good comparison.
It does not matter what currency you are using, it only affects what currency your CPM will be displayed as. If you use U.S. dollar as cost, your CPM will also be in U.S. dollar. If you use euro... well, you get the drill.
I also recommended using the CPM calculator to make sure you get your numbers right.