Considering how much has been spent on the F-35 project vs the fact that only 162 have been 'delivered', it works out to be just over $8 Billion per unit...
Of course, none of the 162 delivered are actually combat ready, but rather only usable for testing and evaluation. Oh, and they won't even have all the promised features until 2022, at the earliest. Even then, they'll still be experiencing some pretty spectacular problems such as the fact that the helmet is so heavy that any attempt to eject would snap the pilot's neck; a MiG from the early 80's can trounce it in a dogfight; overly-complicated power-plant causing numerous engine failures and maintenance headaches; the designs have been modified so much that there are very few parts actually compatible between the three variants...