This article only applies to Data Studio and assumes you have already set up the custom conversion data blend as described here: How to Query for Facebook Ads Custom Conversions
For other Supermetrics tools, there's nothing special required. You just need to create custom column next to the data with a formula that takes the "Amount spent" value and divide it by the "Actions" value.
Contrary to the current Google documentation for Data Blending, you can actually make custom calculated fields from the parts of the blend. This is done with a "chart-specific" calculated field, and is used to create the Cost per Custom Conversion metric that doesn't natively exist.
- Select your report object with the data blend.
- You must have both the "Actions" field and the "Amount Spent" field within the blend to do these next steps. If you don't have those in the blend, please add them like in the example setup.
- Click the Add metric option in the "Metric" list.
- Click CREATE FIELD at the bottom.
- If the option to CREATE FIELD is greyed out: Edit the base Facebook Ads data source being used in the data blend to enable field editing.
- In the pop-up dialog, give your field a meaningful name and add the formula: Amount spent/Actions
This will be the "Amount spent" from second data source in the blend and the "Actions" will be from the first data source in the blend.
Note - if you changed the field names, use the appropriate fields to create the same basic formula.
- You can change the data type and/or display format of the value if you want - this is optional.
- Click APPLY to save the field.
- You now have a Cost Per Conversion field that works with your custom conversion action values.
Because the data blend uses a filter, there are chances that due to how Data Studio behaves with filters that some of the data may be slightly off. This will mostly be seen with scorecards and summary totals than with tables with separate rows.
The custom calculated field is ONLY AVAILABLE in that report object - that is because it's a chart-specific calculated field. If you want to use the same setup in another report element, you have to create the custom field again in that chart/table/graph directly. There is no way to do this as a global field.
You cannot create a global "Cost per Conversion" calculated metric in the base data source and use it in the blend. It will be wrong.