You can definitely build your own, many people do, there is a lot of work put into these thing and yes Winnebago makes a profit to do it for you. They are by no means ripping anyone off, good luck finding a 4x4 sprinter Class B for any less brand new. Good luck finding a 4x4 sprinter shell to convert, they are in demand.
There is nothing innovative about working with aluminum. As a technically proficient person with a shop I would be reluctant to make full cabinetry and bed frames out of the material because things are just trickier compared to wood.
Savvy of Winnebago to pivot their decades of experience in making things light to a new generation.
I find it hard to believe Winneago adds $110k of value to the van.
FWIW, if you want a camper van (and not a full RV), Metris conversions are a solid option. PeaceVans (Seattle) has two nice builds - a weekender (bed, pop top, no kitchen nor bath) or a camper (bed, pop top, kitchen, no bath). About $65k for the former and $85 for the latter. Using one of these (or a 40-year old VW Westfalia) limits you to campsites with toilets (or roughing it a bit). The Weekender build is available through your local M-B dealer (via an official up-fitter arrangement).
2-6 year old class B's (vans) usually sell in the 40-80k range, depending on the details.