Apart from Blissmobil, whose range it seems is expanding, everyone else I've looked at (in Europe) makes what you ask them to.
So you have to pay someone to design and make every piece as a one off unless you do it yourself. Even reasonable labour costs plus reasonable parts costs soon add up. And if you start on a new full size truck chassis that starts out expensive from day one.
Hydronic heater, compressor fridge (and freezer?), calorifier, (custom?) water and waste tanks, lithiums? The list price of all this stuff is easily findable, but putting it all together so it works reliably and fits somewhere while leaving a space you can live in is perhaps hard to do well. All of that from one company? Today our build only has an empty box mounted on an old truck with electrics lined up and that's I think twelve companies in three countries so far, two years later and having previously lived in a Mog camper for three years so we kind of knew what we wanted (and didn't).
Outbound windows have prices on their website, a KCT quote for similarly sized windows were at least twice of Outbounds. You could buy an old camper to get the doors and hatches but they won't insulate or lock as well as decent ones and then you may get condensation on the inside because that will be the thinnest wall section. No problem for the odd weekend away. Crawl through hatch, gas bottle hatch, two for the "garage" (plus a third to get our spare spare tyre out), two insulated access holes down to the gearbox and back of engine, four windows around the dinette, two for the bedroom, one each for the bunk, kitchen and bathroom. One only tiny roof hatch in the bathroom, so another hatch in the wall the opposite end to the main door. All windows with security covers, blinds and mossie nets, net for the entrance door. All then painted body colour, delivered and fitted. That lot almost cost as much as the box. Then a one off sub-frame, after the old truck bed is disposed of, chassis altered to suit and made good. Spare tyre mount on a chassis extension. Three small storage boxes, made square but to suit the space available and cost a lot. Reversing camera. Second fuel tank (new custom tank several thousand, new take off from a truck needing a bigger custom tank €400 (and losing maybe 100l of capacity that a custom tank would have given for the same chassis length), but new mount, feed and changeover valve) etc etc. And etc etc etc. Plus a bit. On a truck from the 70's with no electrics to cause problems.
If you want teak, corian, KCT and a new, big, powerful truck brought together entirely at one point of contact who will advise and steer you, and put bubbly and chocs on the pillow on collection then more than half a mil seems where you will start at without big mark ups IMHO
