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: 
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. 

:)
It's only logical it was made from a dying star, since if it was not extremely heavy it could not be a planet busting weapon - as it is in the comics. 

My explanation is that it is made of a dying star, and Odin's enchantments allow it to weigh only 42 lbs for the wielder but still hit with enough force to bust planets. 

Also the weight is controlled by Thor's mind, so when he is holding back he can hit normal people without killing them, but when fighting gods and beings like Galactus he can hit with planetary force.