The cost is, alas, high in almost ALL early aircraft development stages but, for F-35, they are already tapering towards the approx. $80-90M per aircraft (F-35A) forecast for Full Rate Production from 2019+.
The reason LM has been bunged a $1B is bcause they can't agree a cost with the DoD for the next 2 tranches - a year late. So to start quoting airframe prices, without add-ons, is what might be called flirting with the actualite.......