There is obviously an added cost from the third-party camper manufacturer for taking the van/truck and converting it into a camper. I would see a 20-30% markup over the van/truck price as completely fair for a camper-van. Maybe even a 50-100% markup in the case of a real luxury RV like a top-tier Hymer or Winebago.
The problem is that for older, used campers in particular, the markup is often much higher than that. Van’s that should be in the junkyard based on their mileage and age are still being sold for 5-figure sums simply because they have a camper build. It’s a very emotionally-driven market, and therefore doesn’t make much sense.