I can’t speak for what might be common, but in my case, I got charged the standard shop rate for their shop; keep in mind that includes their time to process the files and create the tool paths plus machining. Seems like that would be a relatively common approach. In my case, since I knew him well, I came in and helped load and pack parts, which saved time. Around here, the shop rate is typically in the neighborhood of $75 – $85. I did 16 sheets in two hours of software time and two hours of machining time, for example.