We just migrated to the new API version (from v4 to v5). During this move following fields have been deprecated: Impression Cost and Click Cost, as they do not exists in API anymore.
We have also deprecated the field Cost which was a sum of all the other cost metrics, to instead only use Campaign Cost. The reason for this is that we found identical values for both fields, however, the field name used in the RTB House interface is Campaign Cost so we're hoping that this will be clearer for you as a user.
For more information, please see the article What data can I get from RTB House? or reach out to email@example.com.