Where is he buying those parts? As a rifle builder of 20 years, it is common to only see 10% margins on parts. That is far from "substantial" in my book. Accessory parts can often have 20-25% margins, but those are accessories and not the main components.
The action and stock/chassis choice can greatly affect the cost of a total rifle.
Defiance Anti action $1600
Proof Carbon barrel $800
Manners LRH mini chassis $1170 (may include bottom metal, depends on the vendor)
Trigger Tech Special $240
Hawkins HTR DBM with mag $325
That's $4135 for parts alone. With a mini chassis stock you trade out the cost of bedding. You still have the cost of chambering and muzzle work, then cerakote and assembly. There are hours of work associated with that in labor.
Can you build a budget rifle cheaper? Absolutely, but the question wasn't really about how cheap a rifle could be built. The question was about why one rifle may cost more than another.
My shop is currently working on producing a rifle we are calling the "blue collar rifle". We have our own feature rich custom actions, using a Proof carbon fiber barrel, lightweight carbon fiber shell stocks, DBM or Obendorf bottom metal. We want to make the rifle as affordable as possible while maintaining high quality components. We're still looking at a $3250 retail price point. That's $1300 cheaper than our normal rifle build so I consider that a "substantial" discount. It will still be way higher in quality than a CA or Fierce rifle.
Sorry, Brother, I could not resist. 
