A friend of mine recently started working for a company that builds such houses (Timber frames for the structure,
straw bales with clay as walls).
The costs are just around the same as for conventional building... the materials may be cheaper but the amount of manual work offsets that.
As for building it yourself: Sorry you can't
The people who started that company had years of trial&error to build up their knowledge, because when they asked old people for recipies it failed miserably because the soil composition differs from location to location. The other reason why is difficult to build by youself, is that in most climates you have a very slim time-window to finish the job, so you need people to help you.
If you want to build it youself, i highly recommend you to not rely on knowledge from
Books and the Internet, but to get in touch with people who offer hands-on workshops so you can gather practical experience before starting you
project.