Re: Where does the money come from?
To answer your serious question seriously, Bitcoin is in a weird state. Some people want it to be money. Which works best by having as stable a value as possible and being useful to buy things. i.e. a "medium of exchange" and a "store of value". Call these people users.
However some people see Bitcoin as an investment. They want wild swings in value. Well OK, they probably don't want it to go down (unless they want to buy more of it) - but they do want the value going up continually. They're your investors.
A worry for Bitcoin is that a large chunk of the miners might sabotage the use as money lot in order to keep dollar values high to maximise their investments (in both mining rigs and coin). And by doing so, kill the golden goose.
Sometimes your user/investor might be the same person. I've seen some Bitcoin defenders trying to make the "it's better than fiat money" argument, but then being unable to stop themselves from smugly saying, "and look how much the value went up last week!"
So the dollar value of a Bitcoin only matters when you convert one into "real money". And I believe turnover in these exchanges is quite low. So the price is set by the next person in line willing to buy.
So the answer to your question is that like any investment, it's only worth what someone is willling to pay you for it now. Which is why it once went from being worth over $1,200 to around $50, in about 2 days.
However, again, if it has the ability to survive long-term then today's worthless asset might go back to having the same value as yesterday if you're willing and able to wait long enough. Half the banks nearly went bust in 2008 because they were holding mortgage backed securities that nobody trusted. So they went from being worth something like 98% of their face value to nothing overnight. Nowadays, those mortgages are mostly still being paid off - and so most of those assets are still probably worth 95-98% of face value - though I don't know what price they sell for.
So, in the same way (and to undermine my own anti-Bitcoin argument), if the Bitcoin economy still continues - it doesn't mattter so much what they're worth in dollars. If people will accept your Bitcoins for stuff, and can spend those with other people, then the Bitcoin economy could conceivably continue forever. I don't know how big the Bitcoin economy is. Or whether people use them like a currency by pricing in Bitcoins, of if you're always charged the dollar value at the time of transaction. That, and whether there are enough non-criminal uses to keep it going, will determine if Bitcoin lasts.