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: 
As I pointed out in my answer [here](https://scifi.stackexchange.com/a/54581/20774), you're misreading the passage. The blacksmith [Eitri](http://marvel.wikia.com/Mjolnir) and his assistants Brok and Buri used the core of a dying star as **the heat source** to "*forge a mold with which he birthed Mjolnir*".

This "heart" (which presumably also served to heat the [Uru](http://marvel.wikia.com/Uru) from which the hammer is constructed) was not **inside the star** at the time but rather had **already been ejected** from the star as it exploded or collapsed.

There's an image of them hard at work in [Thor Vol 2 #80](http://marvel.wikia.com/Thor_Vol_2_80)

![enter image description here](https://i.stack.imgur.com/sOIxm.png)

Given that Uru is a relatively dense metal, we can assume that the weight of the hammer (sans enchantments) is around a couple of hundred(ish) pounds.
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.