Some traffic sources able to transfer cost of each click to the tracker. To activate this feature, go to the Traffic sources section and add/edit your traffic source.
You will see the Cost paragraph. Add there a parameter for URL (for example bid) and a token for transferring the click cost of your source.
Example for Popads:
It is not necessary to use the rest of the tokens. Thus, the URL of your campaign for this source will look like:
In order to obtain click cost automatically, select Auto in CPC section settings and choose currency: