Creating a Custom Metric or editing and existing one can be used to do create monetary metrics based on other metrics. It is also possible to either override the standard rules of a Cost Custom Metric or create a new rule based Custom Metric from scratch or the provided template. Below are some common use-cases and gotchas.

This is typically an existing monetary metrics multiplied by some factor.  It is also possible to have different rules for different sources or dimension values using a rule based metrics. For example Client cost to add 10% to the actual cost can be done as a formula:

Sometimes there is a fixed cost based on an event instead of using for all or some data sources. For instance 10 cents per click or 9 dollars per conversion. This can be set up using rule based metrics. Note that the currency will be based on the data-source and cannot be set separately. However it is possible to condition a value based on the source's currency. For example:

Rule based metrics are run at a data source level-of-detail (a row-level calculation). Thus the currency for a row depends on the currency of the source row and is used for all monetary metrics on that row (custom or native)
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If none of the metrics in a calculation have a currency, the result cannot be converted to the currency requested in the query and no value is returned.