Osmium - Worth more than your mom for one kilo, safe in bar form, bizarre properties when ground or smashed into a powder including pyrophoricity (a tendency to burst into flames for no apparent reason) and then making the victims drown in air as their lungs fill with fluid from the action of the toxic oxides (swelling of the lungs and blindness syndromes I guess). Its density is also the highest of all the elements, higher even than uranium, making it effectively alien space-slade. Osmium-based ammunition would have impressive and desirable properties.
Iridium - Worth more than your sister for one kilo, completely safe. Properties equal to osmium without the toxicity, but still very difficult to obtain without mining an asteroid. Often alloyed with osmium or platinum to make safe and nearly indestructible alloys.
Uranium - Worth more than your grandmother for one kilo in a free market, safety complicated. Natural or depleted uranium has limited toxic effects that are considered acceptable for combat troops, and there's no easy way to simulate the fun chronic ones like slowly going insane or having children with birth defects. Acute exposure from enriched uranium or a nuke... it's scary how much of the radiation exposure tables I have randomly memorized, but you should just refer to this free table because it's complicated:
http://en.wikipedia.org/wiki/Radiation_poisoningTungsten - Cheap, inert, completely safe. Very difficult to melt.
Phosphorous - Cheap, horrifying necrosis of the jawbone in very high doses, survivable eye and lung irritation well below that.
Radium - Has no market price but would be worth quite a lot if it did, standard radiation sickness, plus your body thinks it is calcium and gradually turns your bones to glowing green jelly.