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: 
So here is a pretty good Vsauce video on this.

To summarize - Thor's hammer wasn't made OF a dying star, just IN a dying star. However, if it were made of dying (in this case neutron) star material, it would weigh about 10 quadrillion lbs; given its density and size, anything within 100ft of the hammer would be pulled in at almost the speed of sound, torn apart with gravitational tidal force in the process. If dropped, the hammer would cause an explosion equal to about 1.3 million Tzar-Bomba nukes, killing off a good chunk of humanity, and sinking through the Earth to rest at the Earth's core.
What if the hammer (Mjölnir) is not just a hunk of metal, but rather a concentration of Higgs Bosons with the "hammer" really being a containment device. I base this on the interaction between Captain America's vibranium shield vs Mjölnir. 

Since vibranium can absorb (or cancel out?) kinetic energy, and Higgs Boson gives something its mass, ("kinetic energy") then the reaction of those two meeting would be an incredible force indeed. 

If it is a Higgs Boson variable containment device, then it would be able to rest on normal object. Maybe people are unable to lift not because it's heavy, but rather it increases the nuclear strong force and the hammer literally becomes nuclearly bonded to whatever surface it's in contact with. Modifying the strong nuclear force would be fairly easy if it's modifying the Higgs field with impunity. 

And in order to keep continuity, the Higgs Bosons could have been collected from a dying star, where the increase in mass would allow for easy harvesting of the bosons. 

:)