Can somebody tell me what is the difference between pbAg and cpm in the object informing about a returned bid?

It is the Price Bucket configured for your setup that means you are using the auto Price Bucket, pbAg is the value that will be send in the targeting to DFP, and CPM is the original value returned by the ad network. You can read more about price buckets in the link below
http://prebid.org/dev-docs/publisher-api-reference.html#module_pbjs.setPriceGranularity
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It is the Price Bucket configured for your setup that means you are using the auto Price Bucket, pbAg is the value that will be send in the targeting to DFP, and CPM is the original value returned by the ad network. You can read more about price buckets in the link below
http://prebid.org/dev-docs/publisher-api-reference.html#module_pbjs.setPriceGranularity