Iridium is a metal element, atomic number 77. There are two stable isotopes, 191 and 193. Symbol for Iridium is Ir. It is one of the densest elements known. Much more info in Wikipedia entry for 'Iridium'. No cost info given.
This site here would give you a idea of how much it may cost http://www.cellhire.co.uk/satellite/iridium-purchase.aspx. It can also depend on how much you use your phone so thats just a rough estimate.
Lead has a higher atomic number than iridium, which means a lead atom is heavier than an iridium atom. however iridium is denser, therefore if you had a block of iridium and a block of lead,the iridium block would be heavier