fyi - general offshore drilling costs., page-2

You have to also factor in establishment costs ie. the cost of getting the rig onto site. Generally, thats a one off, so the more work you do, the less impact the establishment is.

Not that I worked in Oil drilling, but for decent Geotechnical drilling programs, we used to try an dengage on a per metre basis, so the incentive was there for the drilling company to work faster...