Oh nvm, found it:
> Hey all. Anyone know what an acceptable Volume to Mass ratio would
> be for an asteroid? I realize it varries substancially based on what
> kind of ore is in it, but I was just wondering what an average might
> look like. (Kinda like how cargo is assumed to be 1:1 by default,
> unless specified otherwise.)
Far Trader is mostly right. When it comes to materials, asteroids are generally either rocky or metallic, so you'd expect density to be about 3300 kg/m³ for the rockies or about 7900 kg/m³ for the metallics.
However, a lot of asteroids seem to be "porous" in that they have lots of spaces inside them, probably because they are actually "rubble piles" - lots of smaller bodies held together by their very low gravity and covered in a surface layer of dust. This 'porosity' decreases the density significantly, pushing it down to as low as 1300 kg/m³ for a porous rocky asteroid. Ice can lower the density too, but if an asteroid contains a lot of ice then it's probably better to call it a cometary body.