Costing information is sent to the device during the authentication process. It's in the DevResExtGetAccountMap message. That message contains an Account Map consisting of 4 sections: Copy, Print, Scan, and Fax. Each of those contains a DocumentAccountList that is an array of cost data for each paper size that the machine supports, for several color options, and for simpex/duplex. There is quite a bit of data in total.
When the job is complete, the device sends back the cost for the job in the DevReqExtNotifyCounter message. There's a Counter element in that message that reports all of the counters that the machine maintains for a job (number of originals, number of prints, copy total, etc.). There's also a DocumentChargeList for each type of job that was executed. Within this element is information about the media type, the color type, the number of sides, the document count, and the charge for the job.