The cost of drilling a horizontal well is about twice as a tranditional vertical well. However, oil/gas industry is a system engineering. Honrizontal wells have their cost reduction advantages. It reduces the number of wells needed signifcantly. I think one horizontal well can cover 2 sqaure miles, which needed 3-4 traditonal vertical wells in earlier times. This leads to several cost reductions. First less pipes and equipment needed on surface. Second, one well can produce more gas/oil. In terms of cost per barrel, horizontal well gives $3-4 in comparison with $7-9 for vertical one.
Horizontal well also reduces the developing cost based on the same reason. The finding cost is about $3-4 per barrel in comparison with 6-7 per barrel for vertical wells.
A development well is used to explore the reservoir production. They are experimental wells and in a small number. It should not affect the total extraction cost very much.
