I would pay $21k for one of these in a heartbeat. I use a Polaris 4x4 and they can go for $8k-$9k with all the bells and whistles. Four stroke snowmobiles can get up to $13k for double ups with hydraulic adjustable suspensions, heated seats, etc. To have fun in one of these and have a sandy beach as my backyard somewhere on the shores of the US would be a dream come true for my wife and I. These look like a blast!
What about the seals involved when talking sand and salt water.
I am a mechanical engineer for a defense company and we have a lot of sand and salt requirements for environments throughout the world. The engineering costs to meet these requirements with wind dynamics as well is very expensive; chit is blowing everywhere into the critical components and those components need seals and protective hardware. Remember they are not setting up an assembly line like Ford Motors; there is a huge production cost reduction when volume is involved.
Your welding process alone can go from hand fixturing and welding to robotic welding stations if quantities justify it. These are hand built custom machines.