Design included with purchase contract. We, Maxwell and customer, can work together to design your cheap dream home. 
I suggest to go with a 20ft.x20 floor plan, one story. We can do up to two stories. A 20x20 (400sqft) is enough space for about 3 residents. At that size, we can plan needed attributes to structure, and after that there can be auxiliary attributes planned like yard wall, fence, driveway, gate, garage, etc.

A 20x20 has a perimeter of 80ft. At 8ft high per story...
* 1 story would have 640sqft of perimeter (vertical)
* 400sqft floor
* +400sqft roof

The perimeter vertical square footage (siding) can be described as one of five parts that comprise the complete  section of wall,
(1) exterior siding (sheet metal)
(2) framing (lumber or steel pipe)
(3) hardware
(4) insulation material
(5) interior wall
All those together make a wall, inside and out. Sheet metal is common in 2x8.
Estimated 2x8 interior/exterior segment cost:
(1)$16
(2)$10
(3)$4
(4)$8
(5)$7
(Total) $45
80ft perimeter would require 40 of those segments.
40×$45= $1,800
The insulation could be optional (-$320), although it might be an interior room difference of 10 degrees, (10 degrees colder inside in winter time and 10 degree hotter inside in summer time)
Your house could have 10 doors, however it must have at least 1 door. I suggest 2 doors and 3 windows.
Estimated door cost $150.
**Locksmith Loophole We don't need a locksmith if the door knobs have different keys and are not mastered togethered. You can pick out 2 door knobs called "entry door knobs", or I will select them. (In case you want nobody to have a chance to copy your keys)
Estimated window cost is $75. 
2 doors and 3 windows together have a material cost of $525. Together, they would subtract about 3 wall segments. (-$135)
Brick(clay), block(concrete) walls require mortar and rebar which is an option for a different kind of wall. If we add plaster to concrete, the concrete will be more waterproof than without, because dried concrete is slightly sponge-like with water unless it has waterproof additives.

Attic science: If the sun is 150 degrees hot on your roof, under that it might be 100 degrees, so attics are designed to separate roof heat apart from indoor air. Similar to insulation, attic is optional. A high room window with an electric fan would do a similar job as an attic.
Interior floor could be concrete, concrete/carpet, tile, concrete/indoor-wood-deck (elevated about 8inches), or concrete/pvc-linoleum synthetic flooring. 
Concrete cost is $4 per 80lb. bag. One bag makes about 2sqft at 3 inches thick. (2×1×0.25ft)
A 400sqft floor requires 200 bags. (400÷2=200) ($800)
The roof could be same composition of wall segment (flat) or plus extra segments for pyramid-like. 
450sqft x (8×2 panel=16) =28 segments ($1,260)

Rough estimate of 20x20house with front door, back door and 3 windows is...

$4,700

That leaves us $2,800 left in budget for exterior extras or other extras. ($7,500 - $4,700 = $2,800)
$4,700 + $2,800 extras +$7,500 labor = $15,000 house.

Need a solar powered ice cold air conditioner? No problem. 
If you buy electric utilities, you can buy wind turbine appliance from me that will plug into your ac outlet so that the electric company can pay you credit for kilowatts generated.
You don't need electric company utilities although your solar panel system is going to need to be large to power freezer, to power fridge, to power stove, to power oven, to power air heater, to power water heater.. 
Quick estimate full solar panel system Ten 100-watt panels (1 kilowatt per hour, solar battery bank, solar electronics, assembled on small trailer wheels $2,500 cost.
Wind turbines are not recommended for applications without ac outlet grid-ties.

Need a large rain water catcher? We could order one and bury it or put it next to your house or in your yard.  Say 500 gallon container, Rain water is good to wash clothes, shower and almost good enough to drink without filtering. Pvc, also known as vinyl, is a terrible toxic plastic (cheapest), common in American water pipes. Instead, we can do non-toxic abs, aluminum, or pet plastic, which is common beverage bottle plastic for water storage.

We do not want to use poison. We should try to use non-toxic materials. 
Toxic materials are pvc, pesticide treated lumber, some concrete, typical exterior paint, typical high strength glue, and any chemical products that are not non-toxic.

Want a custom house that is 100% non-toxic? No problem. Your new house can be near-totally non-toxic or mostly non-toxic.
* 100% disclaimer, non-toxic is determined by manufacturers of products. SimcrimeCom does not guarantee any purities of 100%, where such purities are too difficult to determine. For example, your house could be built entirely of 100% stainless steel, a non-toxic material, however it would be difficult to determine that one or more components of the stainless steel products did not have a speck of toxic contaminant in it when it was melted. (Then it might be 99.9999456% non-toxic, not 100%.)