Well, the actual figures are clearly Brcm confidential, but ballpark figures should give some idea. It does depend on the particular licence a company has with ARM. Brcm have an ARM A7 licence so they wouldn't need to buy another one (that's going to be a couple $M at least), so that just leaves dev costs and mask, so $1.5M to $2M. A company without an ARM licence for A7's would be in the region of $3M-$4M. Note, this is just to adapt an existing chip.
Chip industry is an expensive place to play.
Thanks for the info! Is it possible for you to give us a floorplan of the bcm2835 and bcm2836?
I am really curious about the design of the chips and how the layout has changed between them.
I'd rather not have to get some fuming nitric acid and start decapping and most likely delayering these things.
(I assume there is a power grid on top so that will make things a lot more difficult if not impossible without a ton of specialised equipment).