The Funnel API, BigQuery and S3/Redshift integrations contain some metrics prefixed with common- and other that start with the name of a specific ad platform.

The common- metrics in general reflect the Funnel view of what is the most common interpretation of that metric across platforms. For "cost" that is usually the total cost/spend for the advertiser regardless of wether it is based on impressions, clicks or conversions of some type. In most cases you should be able to work only with common-cost as a way of aggregating and comparing cost across all ad platforms.

When is common-cost different from adwords-cost?

But if you sum those rows together (in your own Data Warehouse, or in Funnel using a custom metric or dimension, the totals for those columns would be 190 for common-cost, 90 for facebook-cost and 100 for adwords-cost. 

I'm missing the cost for a specific ad platform

Going forward Funnel aims to provide both the common-cost and ad platform specific cost metrics. In some cases older integrations may not have them, for example platforms that defines cost as Clicks or Impressions multiplied by some fixed factor. In most cases you should be able to work with common-cost at the right level of detail (see above), and otherwise let us know if there is one you are missing. 

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