Why are motorhomes such a ripoff compared to caravans?

I know you have to pay for the van the conversion is based on when you buy a motorhome, but if you look at the price of a fiat ducato van for example, new, at about 14000 or so, an autotrail motorhome based on the fiat chis as seen on one website is 47000! Does it really cost 30000 to convert a van into a motorhome? If you got a caravan of the same spec and same size living space it wouldnt cost 30000. Even taking into account the cost of the vehicle the motorhome is based on, its a far higher price. Any ideas on this one?