You likely wouldn't save half, I would guess ~20-30% provided you do the majority of the assembly. The era of Sears homes benefitted from lax or non-existent building code (especially for electrical/plumbing).
You may also have problems selling the house in the future being it was not built by a professional. In the near term you may be unable to get financing (if you do, it will likely be much higher than a standard mortgage rate) as there is no contract or guarantee the house will be completed; they are just paying for materials.
One of the biggest costs will be the property. You need a lot zoned for residential housing with available utilities (sewer, water, gas, electric) or alternatives (septic, well, propane, wind/solar).
The lot has to be surveyed, plans approved by the building commission and permits paid for.
The foundation plan will have to be drawn up by an engineer or architect because frost depths and local regulations vary. An owner could maybe pour a slab but likely couldn't do a basement on their own; either is best left to professionals.
Now the person has to assemble the house. You could get by doing much of the work alone but would likely need a helper for various stages throughout the project.
Most of the materials are cut but you still need some tools which would be another ~$2000+ expense; ladders, air compressor, nailers (framing, trim, roofing), drills.
When it comes to the roof, especially if a 2 story house you will have no choice other than hiring a crane to set the trusses. You'd likely have to hire a crew or at least have someone experienced working with you because this part is dangerous even for professionals. After the trusses are in place you could sheath/shingle the roof yourself.
Once the shell in complete you can move inside to finish the house. This is where your local laws will make the biggest difference in price. Some will allow homeowners to run plumbing and electrical themselves, others require all work be completed by a licensed professional.
After all the systems are installed (electrical, plumbing, gas, hvac) you can start finishing the house. This part is most conducive to the DIY process as it is all aesthetic. Drywall, flooring, trim, kitchens/baths.
TL;DR -- It would still cost a significant amount and require a huge amount of labor from both the owner/builder and professionals.