Question: All of the sites I looked at said "heavy" or "a lot" or "42.3 pounds." Mjolnir was forged from a collapsed star (a white dwarf or a neutron star). This means it is insanely dense. 

Worthiness aside, given its size (as seen in the movie) how much should it weigh?

Perhaps I should clarify. Given the average density of a collapsed star, if you took enough material to construct the hammer, how much would that item weigh on Earth?

Answer: 
The magic of the hammer makes its weight arbitrary. Consider that weight is the force on an object due to gravity, that means that we have to take into account the mass of the hammer and gravity(a constant). However, the hammer doesn't adhere to physics as we know it. 

At one moment it can be wielded by Thor, set on a table without breaking the structure, and fly through the air. At the next moment the Hulk himself, who we know to be exceedingly strong can't lift it up. If we attribute these instances to its weight changing, then it can obviously be any arbitrary weight necessary. (Technically we don't know if there is an upper bound but as it's **magic** it is safe to say there is, in so far as it is convenient to the plot). If we attribute them to some force other than a change in weight, the weight still becomes inconsequential, as it still performs the same way as if it did not have weight. 

To reflect on your edit:

From the [transcript](http://www.imsdb.com/scripts/Thor.html)

> 
> Forged in the heart of a dying star, from the sacred metal of Uru...
> 
> 
> 

I think this is saying that the **location** of where the hammer was forged is in the "heart of a dying star", not that it was forged **from** the dying star. It is a common idea in a lot of literature to say that an object (sword, spear, shield, hammer) is stronger when forged in a hotter furnace(not being a blacksmith I can't attest to this but it makes sense), I imagine that this is just a matter of Odin indicating that the heat required to forge it came from a dying star.
Mjolnir, Thor's hammer, was crafted from uru, an element native only to Thor's realm, Asgard. It being a heavy metal, logistically speaking, I'd say it weighs at least a good 50 pounds or so, but its weight is not the key.
Mjolnir, from the day it was begotten had a spell cast on it, by the All-Father Odin, literally the spell from the movie *Thor*, stating only the worthy may wield the hammer, and the beholder will have the power of Thor.

So staying precise, in truth no one besides Thor himself can lift Mjolnir, no matter its weight.