As I understand it, the Air Force did not try to incorporate new cutting edge technology in the design but stick with already developed tech. The R&D should be fairly cheap, and easy allowing for the plane to be built for around the price and time frame announced (adjust for inflation).
If however during the development phase decisions are made to increase capability beyond the initial design then all bets are off
That is because this is evolutionary, not revolutionary.
The B-2 was a revolution when it came out, and as with any first generation aircraft it was more complex and expensive then planned. But the lessons learned from that project are going to make the going on this one much easier. This is true in everything, from battleships and aircraft carriers to missile defense systems, tanks, and conventional aircraft.
Then I can't help but wonder why an entire new air frame if it is just a follow-on... Off the top of my head I can't think of a 'new' project that didn't try to redesign the wheel. Perhaps you can think of one.
You can't?
Well, it is safe to say that every carrier built after WWII is simply an evolution and not a revolution.
Prior to WWII, most carriers were originally constructed as another vessel, then converted either during or after construction. This all changed after the war, when these conversions had all either been sunk in action, or retired at the end of the war and replaced with those specially built for the role during the war. And once the angled flight deck and CATOBAR became the standard, almost all carriers followed the same pattern.
The same with Battleships, from the end of the side blister until the last ones were built. Thick armor, three 3 gun turrets, etc, etc, etc.
Unless something is "first of a kind", almost everything that follows is simply evolutionary. Pretty much every cargo aircraft in the military can trace it's roots to the C-47. And the AC-130 of today is little more then an upgraded and improved AC-47.
Other then the F-117, B-2, F-35 and F-22, I find it hard to really think of any "revolutionary" major military equipment in the past 40 years. Not counting the low or limited production equipment that were largely experimental and never intended for large scale operational use. Like the USS Pegasus class ships. Only 6 were ever built, all but 1 only 11-12 years old (the oldest was 16 years old).