1890$
You children have no vision! Damn, I am typing this sub 2k$ number and think: "I am getting rather pessimistic without any good reason to be, lately"
The upper bound for the value of 1 BTC in this year's USD is 1-5 million (given finite earth, etcetc,). Realistic positive long term scenario is 300k. If the whole crypto or BTC fails in some unforeseen way, it can be worth so little that it is impractical to trade.
Let's put the possibilities in log scale (price of BTC in current USD):
10^7 = 10 million. Total value of bitcoins would be on par with total value of the Earth. This is not possible and this scenario is therefore utopistic.
10^6 = 1 million. All bitcoins would be about as valuable as the M1 monetary stock of a top-notch fiat currency such as the USD. This is in the realm of possibility, but clearly the most positive scenario imaginable, and therefore unlikely.
10^5 = 100 000. Bitcoins would be more valuable than all the silver ever mined, but less valuable than all the gold, and much less valuable than many fiat currencies of today (total value about 1000 billion).
10^4 = 10 000. This will be reached ??
10^3 = 1000. This will be reached ??
10^2 = 100. This was reached 4/2013.
10^1 = 10. This was reached in 6/2011.
10^0 = 1. This was reached in 2/2011.
10^-1 = 0.1. Minimum value that bitcoin has ever traded in an exchange. At that time there were no actual uses for the currency.
10^-2 = 0.01. This would render all the bitcoin economy worth about 100,000 USD. As this value is spread between millions of users, nobody has a big enough stock to have any incentive to care about bitcoin, and the exchanges and eventually even the network would likely grind to a halt. This is therefore an impossible long term scenario. Altcoins die if they cannot overcome this barrier or fall through it.
The market is very volatile because market participants need to assess probabilities to scenarios, which are not close to each other in outcome. In contrast to stock market, where there are reasonable bounds as to what a certain stock's value might be, with bitcoin there are only absolute bounds, 8 orders of magnitude from each other. If the market would reach consensus that in 1% probability, bitcoin's value is 1 million (and 99% probability it is zero), the price would explode to the upside as any price short of $10000 per bitcoin is a bargain with positive expected value.
Bitcoin is not in a bubble but in a genuine price discovery phase. Not longer than a few hours ago, I told that I expect bitcoin to rise to 4 digits before next summer. My friend's guess was $15. Both are preparing accordingly.