We could look at the BitCoins that can be found as the resource. Everybody is going at it as hard as it as they can, driving the costs to prohibitive highs.
No, not "everybody" -- if it is too expensive for you, then you stop doing it. Only the people who can generate most efficiently will stick around, and they'll stop "going hard" when the cost of adding more generating hardware is more than the value of the bitcoins they get.
Hopefully bitcoins will become more valuable over time (that will happen if people find them useful/valuable).
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Other than usual in a ToC the resource can not be destroyed directly. I don't know yet, but perhaps it's usefulness could be greatly decreased, though. For example if the same computing power is needed to execute transactions, the transaction costs might be too high in practice (I admit I don't know how it works yet - is it easier to verify small transactions than big transactions?).
Mostly no, total value of the transaction doesn't matter. A large-value transaction that is made up of lots of little transactions is more expensive to verify, but so is a small-value transaction made up of lots of tiny transactions. And for both, if they're big enough they'll have to include a transaction fee to get accepted.