While cruising The Oil Drum I found this article about Thunder Horse. It details in pretty plain language the reality around the development of the field. What started out as a $1B dollar development pretty quickly rose to $5B after Hurricane Dennis. All for 300K bpd of oil. And that is in the Gulf of Mexico, which is challenging, but somewhat less so than the Atlantic.
Think about this as you get excited about BP's Tiber find in the Gulf (similar environment) and Brazil's Tupi field in the Atlantic.