It’s not difficult estimating costs for small houses. Simply add up the number of each main component and multiply times a realistic cost. Then add about 5%-10% extra to cover unforeseen expenses. Use current, local prices for most accurate results.
Here’s one article that explains how to estimate the number of bags you’ll need.
You can also do a per square foot cost estimate. This method is not as accurate as a detailed cost breakdown, but does provide a rough estimate.
Here’s one example using dirt cheap building techniques. $10/sq. foot is about as low as you can get using simple, low cost materials and methods (earthbags, rubble trench, earth plaster, locally harvested wood, recycled materials, etc.). So a house of about 300 square feet would cost around $3,000 not including land or labor. (300 x $10 = $3,000) Then add any extras you may want: radiant heat, better windows, tile counters, etc. to get a more accurate cost. The cost will be significantly higher if building in areas where you have to meet building codes. Seek out remote, rural areas with few or no building codes to minimize costs.