Moreno is worth more to the Broncos than his free agent sticker price.
He has been on the field with Manning for hundreds of snaps the past year and a half. That's worth something. Those reps develop his anticipation and awareness. There are plays where he would pick up a blitz, etc., or anticipate what Peyton wants--and a back without that experience might not. At this stage in Manning's career, that seems an important commodity.