Relative specific gravities and melting points (degrees F)
Platinum - 21.4, 3220
Gold - 19.3, 1945
Lead - 11.35, 621
Silver - 10.5, 1760
Copper - 8.96, 1983
Nickel - 8.8, 2647
Brass - 8.4 to 8.7, 1710
Iron - 7.9, 2797
Stainless steel - 7.5 to 8, 2750
Zinc - 7.13, 787
Cast iron - 6.8 to 7.8, 2150 to 2360
Lead is obviously the most effective bullet in terms of density and low melting point, but there are alternatives available now (like copper) which are much more costly and harder to fabricate (not to mention lighter). Raw copper is 200%+ more expensive than raw lead.
Zinc would seem like a likely candidate, and is only 1/3 the price of copper (same price as lead), but would make for some pretty lightweight bullets. (Post-82 pennies are 97.5% zinc). I have no idea what a pure zinc bullet would do to the bore, but chances are that copper-plated ones would be fine.
Iron and steel are both relatively cheap, but are relatively light and have really high melting points as well. Not to mention, shooting steel would seem to be really hard on a barrel unless it was plated with copper or something else.
I'm not advocating any certain position, just throwing out some facts. I like my <relatively> cheap lead bullets
A ban on lead would make it hard on all of you who fabricate your own, even the lack of wheel weights after 2011 is going to drive up the price