Maybe in these economic bad times, gold is your pick?

Pound for pound, everything weighs the same. If you mean density, the most commonly used material is uranium. There are higher density metals (trans-uranic), but are highly radioactive and most have very short half-lives.

I suspect that 5 pounds of lead will fit the project’s budget.

If you have the cash, there are much denser alternatives. Five pounds of iridium, for example, is about $51,000 US.

Grams per cubic centimeter
Osmium 22.61
Iridium 22.42  
platinum 21.4
plutonium 19.8
tungsten 19.6
gold 19.32
uranium 18.9
mercury 13.55
lead 11.36
Silver 10.49

You want Tungsten. It is kinda cheap, extremely heavy and you will be able to have babies after handling it.

There you go, I assumed it would be pricier, but @johnpowell has your answer. It looks like 5 lbs of tungsten is only about $67 US

And it’s almost twice the density of lead, so a little over ½ the size.

And I should add that Tungsten is the metal used in projector bulbs at movie theaters. I used to take it out of the bulbs after they were dead. I had a bucket that had about 100 pounds of the stuff. A lot of kids asked for tours of the booth so we would let them look around and give them trailers and projector parts that were broken.

http://i.imgur.com/X801T.png

That big chunk of metal in the bulb is Tungsten. There is about a pound in each modern projector bulb.

Here is a tiny bulb exploding. http://www.youtube.com/watch?v=SVpD8SWzKFM

The ones we used were about four times the size. Changing them was frighting.

The only cinematic projector I ever used had an arc-lamp. That was fun.

The densest matter which can exist (beyond which it would form a singularity) is degenerate matter from the core of neutron (or quark or preon) stars. Strange matter from a preon star would have a mass of 10^23 kilograms per cubic meter; a preon star with the mass of the Earth would be the size of a tennis ball.

Zinc dust is pretty cheap and freaking heavy! Call up Kraft Chemical and request a sample….they’ll might send it to you free. 5 lbs is less than a quart.

For your friend’s project, I would think sand would have to do. Iridium and osmium have the highest measured density of any materials on Earth. Osmium weighs 22.61 grams per cubic centimeter and Indium seems to weigh 22.65 g/cc but the numbers are uncertain and both are classed as equally dense. Whatever the case, the two are roughly 2 times as heavy as an equal amount of lead. However, they would be completely inappropriate for a project where low cost is a primary concern. THey are both metals in the platinum family and are quite rare.

Even lead would be out due to cost. Maybe salvaged concrete slabs would be a good bet.

Osmium and iridium are expensive (Os: ~$380/troy ounce, Ir: ~$725/troy ounce). Pure osmium has the nasty property of forming a volatile and highly toxic oxide in contact with air.

Anyone familiar with the phrase “it’s dirt-cheap” ?

I think you had the right idea to begin with and certainly the easiest and most affordable.

Just head for the beach or the back yard with a bucket and shovel. Problem solved.

mercury

Osmium is very dense (heavy), but white dwarf degenerate matter and neutron star matter is a lot denser (heavier). The robotic spacecrafts are costly, not to mention the mining equipment you will need.

So I recommend earthly lithium which is not as dense (0.512 g per cm^3) but a safe investment because demand will increase tremendously.

@mattbrowne The OP asked for a low cost material, so I would say mining a White Dwarf is off the table. Further, the white dwarf is composed mostly of carbon and oxygen. It’s mass comes from the fact that it has lost the core temperature to support fusion, thus losing the reaction that prevented mass collapse. Only electron-degenerate pressure prevents its total gravitational collapse. If you could bring a chuck of a white dwarf back to earth, it would have the mass of carbon and oxygen here on earth, as it would no longer be under the influence of gravitational collapse.

@ETpro – I know. I just wanted to point out that metal isn’t the heaviest material out there. Even on Earth our technology can produce degenerate matter, although in tiny quantities only.

@mattbrowne Yes, we just can’t keep the matter degenerate for long. Matter is so different from humans. :-)

@ETpro – Yep, it’s kind of difficult to fall in love with a woman made of degenerate matter…

@mattbrowne, oh, the plays on words from that last statement!