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: 
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.
The details of the the dying star are irrelevant. In order to calculate the mass (or weight) of it, you would need to at least know the chemical properties of its metal (Uru). The properties (including its density) are unknown because most samples are "heavily shielded by magic". Density of a metal is determined by its atomic structure.

The dying start is relevant to Mjolnir because of the massive amount of heat required to deform it. Being forged in a dying star would not increase the density of Uru (or the weight of Mjolnir). It might ensure that there are no impurities or bubbles (to ensure pure solid uru) - but the density of Uru would remain the same, be destroyed, or become another element entirely.

Either way, the best we could do to imagine the heaviest possible hammer is use the density of a hammer made of our own heaviest metal (without impurities). 

We cannot know anything about the weight of Mjolnor without knowing more about Uru. All we know is that is a highly unmalleable heavy metal. Might be as dense as Lead or Iron for all we know. 

Like others have said, the real truth to Mjolnir lies in its magic.