A competent DIY'er can build a basic composite panel box 13 feet long by 78" wide (NPR cab width) for well under 5 grand. Pick your favourite appliances, mounting system, batteries, etc., and you have a completed box from 5k (sleeping bag and water bottles) to hundreds of thousands of dollars. I do not believe that it makes fiscal sense to spend six figures on something the size of a child's bedroom, when one can purchase a small house or condo in most North American cities for what many shop built "expo" trucks cost.
If you are paying shop rates for someone to build you custom suspension, spray a show car quality paint job, create custom vinyl wraps or create a Bentley level interior, the cost will simply be exorbitant. Custom work, especially quality custom work, is very very labour intensive and therefore has a high cost associated with it.
Buy a well maintained fleet or company owned used truck for 10K - 15k, invest heavily in any needed service and/or preventative maintenance; now you have a good foundation.
Construct a composite box from Phenolic or XPS foam and alloy sheet or phenolic sheet. Bond the panels and spray the interior and exterior with two part heat cured Polyurea. You can DIY this or pay a shop to do it, depending on your skills and budget. You now have a quite durable, completely sealed interior and exterior.
Add windows of your choice (KCT and Outbound are top of the range, but like most quality products, have a higher upfront investment cost than cheap $hit from eBay or $20 take outs from an RV wrecking yard. You get what you pay for here.
Styrofoam cooler and ice, a trucker fridge from your local gas station or a National Luna - your choice ($ to $$$)
Reclaimed wormwood floors, Quartz counters, Baja Designs LED lights, Dornbracht and Duravit fixtures or bargain hunt and buy from the box stores, again your imagination and budget are the only limiting factours.
Wrecking yard one ton axles, rebuilt on the floor of your garage or ring Torq for a pair of Super 14's and shell out $25k for a pair of custom axles installed. DIY rebuilt junkyard axles should be well under 3k for the pair.
Mix and match leaf springs from various junkyard trucks to get your preferred weight carrying capacity and ride quality or call Deaver, Alcan, Betts, etc and have a set made for about a grand per pair.
Prefer links and coils? Get ready to shell out many thousands of dollars for a fab shop to build a custom link suspension front and rear and order custom coil-overs, bypasses and air bumps from a vendor like King. Dual ORI's are an alternative, but they are about a grand each last time I checked and you need eight of them to carry the weight. Rancho 9000's are $100 each wholesale. What is your budget?
Comfoy junkyard seats with Auto Zone seat covers or perhaps you fancy a pair of budget suspension seats from PRP or Corbeau? Maybe a Recaro Expert M series is more to your liking. They are about 3 grand per pair.
I could go on and on, but you see there are many ways to build a truck. IMO, you could have a very nice truck, built on a budget for something on the order of 30k - 40k if you were able to DIY everything. The more you pay a shop to do or the more expensive your tastes are, the bigger your check book needs to be.
If I had 250K to build a truck, would I buy something like an Earthcruiser, GXV or Earthroamer? Absolutely no way in hell....
YMMV