Custom cost model
When Custom cost model is checked you can fix your costs on the calendar manually. To do this select the cost model:
Then Save campaign settings. And in the Campaign page grid check saved campaign and click Edit Daily costs button:
At this step, you can fix your costs data for a time period. Also, you can specify the type of payment and the cost of the click at which you buy traffic day-by-day. CPC - cost per click, CPM - cost per 1000 clicks.
When configuration is done click Save button.