Where about do you think it ought to be valued? Personally, as a completely casual observer, the current valuation seems incredibly high and unsustainable, but I'll admit that's an uninformed gut reaction.
Paypal had a total payment volume of $145 billion in 2012. Now you can't just say a fair value for bitcoin's market cap is $145 billion because that doesn't take money velocity into account. Some of that volume could be the same dollars floating around. You don't need $145 billion in cash all at once to cover that activity. But paypal's revenue was $5.6 billion. So the amount bitcoin needs to be worth to handle the same amount of business as paypal is somewhere between those two numbers, and even a low $10 billion prices bitcoins at $1000 each.
Visa Inc had a total payment volume of $6.4 Trillion in 2012. Of course in addition to figuring out the actual money velocity you need to remove the credit component since bitcoin doesn't do that. But some sketchy back-of-the-envelope math can demonstrate that a $100 billion dollar market wouldn't be unheard of, with $10,000 bitcoins.
Of course I'm talking like 10-20 years out for anything like that, not today.
Depends what you mean. Are you saying it's overvalued because the current economy of Bitcoin is not strong enough to make it worth $100, or are you saying a Bitcoin ever being worth $100 is already too much? Because Bitcoin has the potential to be worth millions of dollars/unit:
Whether that will ever happen, or if it will happen in the next few decades, it remains to be seen. But the point is a Bitcoin does have the potential to be worth a lot per unit. It all depends on how many people and businesses are going to use it in the future. If you look at it like that, Bitcoin hasn't even scratched the surface of its potential.
Due to the limit on the amount of bitcoins that can ever exist, for them to move a really large amount of wealth around the world they will have to be heavily subdivided, making each whole bitcoin necessarily worth a lot more than $100. Do the maths, it rapidly becomes obvious.