I have a slightly different opinion on this. As always, it depends.. For some manufacturers who produce only, or mainly AV Preamp/processors, it would cost them more to build AVRs that offer the same specs and features (on the preamp/proc part). For others such as Yamaha, Anthem, Marantz, Arcam, then their AVPs should cost them less to make.
Economy of scale in such cases are not going to be much of a factor because they typically share many if not most parts and circuitry. If you have seen many Marantz (or Yamaha) AVPs and AVRs service manuals, you will agree with me. Marantz actually do charge more for their AVRs than their corresponding AVPs though the difference isn't that much, simply because they know they can have a higher profit margin because AVP's potential buyers don't just opt for AVP because they want to use the balanced IOs but because they thought AVPs would "sound better".