When Cleanroom did it, it was anywhere from a little cheaper to a little more expensive. Praxis's process had lower defects with higher, upfront costs. The one quote I heard on price was about 50% more than other suppliers. Praxis and some Cleanroom suppliers issued warranties for their code at specific defect rates. So, I imagine it's not as hard as you suggest to have high quality software. Heck, I think companies saying their stuff is higher quality could get away with less quality than those warrantied products given current state of software market.
http://infohost.nmt.edu/~al/cseet-paper.html
http://www.anthonyhall.org/c_by_c_secure_system.pdf