It's half as much as the development of the entire flipping Space Shuttle. It's more than Apple's entire yearly R&D budget. It's more than the James Webb Space Telescope and the entire Hubble program (including operations and the repairs that were needed post-launch) combined. It's roughly the cost of 8 Mars rovers of the same type as Curiosity / Perseverance.
This is worse, even, than the Joint Strike Fighter program, which at least started out reasonably with only a pair of 1 billion dollar grants handed out to Boeing and Lockheed to develop two functional prototype fighter aircraft (never mind how the costs spiraled later).
Setting aside the question of how much it should or shouldn't cost to develop such a product, it's absolute insanity to do it with a single massive project. The DoD loves to claim in public they're trying to be more agile, and this is the exact opposite of that. This ought to have been done $100 million (or less) at a time with limited initial scope and incrementally expanding goals.
Don't make excuses for this. There is no possible justification.
For reference, an ounce of gold in the peak at 1980 (which was a long while peak btw) was $800ish, an ounce of gold today is $1800. So more than 2x.
So $22B today is more like $10B in 1980.
Which is around 49 billion in 2020 dollars, adjusted for inflation (gold is a poor comparison, it's closer to 5x than 2x).
At some point you have to make a cost-benefit analysis and look at the bigger picture. For example and in this particular case, yes, spending let’s say 10 billion instead of 22 billion for a rougher solution might increase the dead count on the battlefield in the near future, but if those 12 billion that you’ve now saved go to better free education at home (more engineers in the future to design better weapons) or/and to free healthcare (more fit soldiers in the future) maybe it will all even out at a higher threshold.
Given that soldiers are currently getting by with something that works 0% of the time, the assumption that 90% isn't good enough seems highly suspect. Exactly how many more soldiers will be saved by developing a true heads up display versus an off the shelf app running on a ruggedized phone?
This seems like an obvious case of a failure to actually define requirements and make real engineering decisions.
Do you ask why space travel is expensive? It's arguably much simpler than this is.
If a pair of goggles fails, the soldier takes them off.
These are the same people who equipped soldiers in Vietnam with a rifle that jams due to mud. The idea that military hardware has to be many times more reliable than space hardware is absurd.
Either way, if this is worth doing -- and I agree with you that it's a big, big if -- it is worth doing right.
Increased situational awareness has value, but it is a clear case of diminishing returns. Exactly what the cutoff ought to be is debatable, but that the cutoff exists is certain.
If you consider an alternative, just make an app and stick phone on a rifle, sure, that will work for 90% of the stuff you want to do, but it would break and that's bad. A true "heads up display" that you can bet your life on, anywhere on earth, at any time, is just a truly massive undertaking.