Just came back from Laserworx in Pottstown. They did a great job on my MP5 clone SBR. Called in the morning, they gave me an appointment right away, and they did a really nice, professional job while I waited. Total cost for the engraving was $50, which I thought was very fair for such a professional job.
They are very familiar with the NFA engraving procedure and with various firearms, so don't hesitate to bring your job to them. He said he loves doing NFA engraving work so you won't find a more supportive vendor!