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.
If Mjolnir was forged from a dying star or neutron star, than below is an approximate weight. 

5 millilitres of neutron star material has a mass of approximately 5,500,000,000 tons.

The Dimensions of Thors hammer has been estimated at 4x4x8in or having a volume of 128 millilitres. 

Mjolnir can have an estimated mass of 140,800,000,000 tons.