I freely admit to not being an airliner expert, I spend my life working on nothing bigger than a twin turboprop, and historically nothing bigger than a BAC 1-11. So, please treat what I say with that in mind.
It is not necessary to make the bulkhead pressure sealed - you simply need to make sure that the smallest human beings (or their weapons) can't get through. A ballistic fibre barrier could be made comfortably people proof and effectively un-cuttable (and you only really need to make it proof against penknives and hacksaws, nobody's going to get a power tool through even the most lax airport security. This can equally be as leaky as a sieve (and before anybody mentions gas, what's the point of gassing the aircrew if you can't get into the cockpit to take over!).
Costwise, it's hard to say, but I'd guess that you could probably do it for somewhere between £35-50,000 per aircraft. It will probably depend upon the aircraft; for example in a 2-crew jet like an A320, you might need to lose the front row in 1st class to fit the aircrew loo and microwave in. Hardly the end of the world in the current climate, I doubt that you could fill it anyway !
If, as I said, the door to the cabin is interlocked with a WoW switch, then loadsheets, deciding whether to throw drunk pax off before take-off etc, can still be achieved. But, it means that the worst case scenario should become a hijack on the ground, which is infinitely more containable than one airborne.
If I wanted to get something like this done, I think I'd go to Marshalls Aerospace at Cambridge [UK], who are leaders in doing odd things to airliners. Remember Pegasus, the L1011 they modified for the US DoD to fire anti-satellite weapons? People like Marshalls could probably have it certified and flying in 4-6 months if they were just told to get on with it. One the basic concept is sorted on one type (say a 767) then you could probably get each new type going in another 6 weeks or so. Mass production is just a matter of cash.
G