Our all in cost was £8000 but that doesn’t include the cost of the borehole drilling and lining etc. The farmer we bought the land from paid for that and it was £18,000. Our hole is 147m deep. Finding water was a condition of the sale. Also keep in mind there’s no guarantee they’ll strike water or at least a plentiful supply. I know of one self-builder near us who left the borehole drilling to near the end of his build (mental). It took several attempts to find a suitable supply and 5 different companies tried. It cost him tens of thousands and took almost 8 months. He assumed you could drill a hole anywhere and find water because there were a few boreholes near him.