Keep in mind that the bone dome replaces a color holographic HUD in the F-35. I don't know what the price of the HUD and fancy helmet are in a Typhoon, F-16, F-18, etc, but I would not be surprised if together they come close to or are maybe even more expensive than the F-35 bone dome.
Mayhap, but an aircraft only needs one HUD, and it's integrated and harmonised and doesn't need regular calibration. But now you have a separate helmet for each pilot, plus spares for the accidents that occur carrying them around on debts, from Ops to HAZ etc etc. And each has to recalibrate the aircraft system every time you climb into it from the last one.
Numbers wise I'd estimate a threefold increasing in numbers, and a major calibration problem on an ongoing basis.