Well it depends on what you build, and what the prices are of houses in your local town.

If you are to ignore land cost for the moment, then you could easily build a good 3/4 bed eco house for less than £100k. If you are modest in your needs, then a simple small 2 bed, designed well could cost less than 50k. If you clever in your acquisition of materials or finishings then you could bring that price down.


But the whole economic situation is going to stir things up, as oil is now $120 a barrel, and predicted to reach $200 in a year or so, then the price of materials is going to go up n up... as well as fuel bills.
And depending on what happens with the housing market, those prices are falling.

Maybe we need to go really low impact, like the £4000 house, or somewhere between the £4k and £100k.
Then you gotta add the land cost..but hopefully land value will be decreasing while we see the downward trend in the housing market.

Project managing is a great skill.
A badly planned project will take three times longer than expected - a well planned project only twice as long as expected.