What does it cost to build an offshore oil rig?

Depending on the type and specification of the rig it could range from $190 -400 million. Jack-ups usually costs between $200-$250 to build, while the more advanced semi-submersible rigs and others built to drill in harsh environments could range from $300-$450 million.
It depends if you want a new or old one. A new one would cost up to $100 to $400 million. A old one would cost up to $1.5 to $10 million.