How much will an Eco Building cost?

It is impossible to accurately answer that question. It is like asking how long is a big ball of string....costs are all project specific.... dependant on the process, size & complexity/simplicity of brief & design, self involvement or not etc.
Some other inputs:

• Time, cost and quality are an interrelated whole....i.e. the faster you want it the more you pay. If you don't pay enough the quality suffers and so on....
• Straw bale is generally very fast and inexpensive....if you know what you are doing, are well prepared and have a good plan....we have a method of pre-stacking and then pre-dipping the bales in a slip to greatly speed up the plastering process. Can be timely or even disastrous if you don't know what you are doing!!
• Walls typically account for 20% of the total cost of a building!!! So you must also be prepared to save elsewhere.
• With the sandbag system the labour need not be skilled (saving cash)...but the bags / fabric is expensive...the "eco beams" in the walls can be replaced by simple timber ladders helping to cut down on costs. Also instead of using wire mesh and cement plaster you can use a primer coat of cob followed by earthen plasters internally and lime sand plasters externally.
• Cob is a very sturdy and versatile medium but can be very time consuming...though it helps alot if you get a way of mixing it real quick.
• Hemp would be quite expensive given one must import all the material at present.

(Also please see “Is an Eco Building cheaper then a conventional building?” )