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It's not simply a matter of "how much" does it cost. Ask also, how much does it cost not to have it?
If an airplane is down and lost, how much does the search cost? How much does it cost to suffer a repeat event when the cause of the original accident is unknown and unmitigated? How much does it cost fruitlessly looking a variety of possible causes that could be ruled out with recorded data, if available? I'm not presupposing the balance here, but the way technology is advancing, the costs becoming easier to afford.
Not to beat an oft repeated saying, but if you think safety is expensive try an accident. Lives could be in the balance.