There is a misconception that natural building is inherently cheaper than conventional construction. We have probably contributed to this idea by building a cob house for $7,000. While this is not a record for cheapness it comes out to about $25/square foot and that is one-fourth the cheapest contractor rate I know in this area.
The reason our house cost so little is not because the walls are made of clay, sand and straw. It is because, first of all, there was no paid labor. Lin and I were helped by many generous volunteers. Secondly, the house is only 14×18′. Subtracting for the wide walls and adding the space in the loft we have about 300 sq’. The national average is over 800 sq’ per person! This cozy space works for us because we are outside as much as possible. Lin likes to say that her favorite thing about our house is the ceiling and she points to the sky. Finally, our house is cheap because it has almost no systems. A free woodstove, a sink made from a popcorn bowl, and a two burner stove we stole from the camper do not add up to much.
All the most popular natural building systems are really just wall systems. In conventional building the walls, including inner and outer covering, are less than 15% of the total cost of construction. Even if your walls were free you would not save more than 15%. And if you did cob or strawbale with paid labor you would probably spend more than conventional construction. Natural building tends to be much more labor intensive than conventional. Stud walls were created as the fastest way to build and as far as I know they still hold that distinction.
Happy Campers The Creations of Hap & Lin Mullenneaux 
What about a toilet and septic system costs? Water supply?
There are very low cost, sustainable septic solutions. Unfortunately, they are not legal in Iowa and most of the US. I highly recommend the Humanure Handbook as an example of a sane septic system. Flushing our waste down the toilet is a waste of good waste. And many large cities are finding that it is also a great way to recycle other things like drugs which go into the septic system but are not eliminated by it. At our homestead we had to put in a septic tank to be legal. This feeds a constructed wetlands. Total cost about $3,500.
We are not on the town or rural water supply. Our largest home (1,000 sq’) sends rainwater from its roof to 5,000 gallon underground storage. From there water is pumped into the house and goes through multiple filters including UV. I do not have exact costs on this system but it is probably approaching $10,000 including the storage tanks.
For the cob house, Lin and I capture rainwater off the greenhouse, summer kitchen and cob roofs and store it in 55 gallon barrels. We use a Berkey water filter for drinking water. It cost about $250.
I live near Fairfield, in Henry county, and I’ve been thinking about making a rocket mass heater in my basement. Where did you get your materials, more specifically, where/whom would I get clay?
Iowa is blessed (cursed?) with clay subsoil. In a typical field it is a foot down or less. Usually, a yellow/tan color but it can be grey or red. Because the clay content of subsoil varies, you need to make cob test bricks to find the right balance of clay, sand and straw. We highly recommend the book Rocket Mass Heaters by Ianto Evans and Leslie Jackson.