Great graph from ProPublica on where the Super PACs get their money and specifically on how dependent they are on their largest donors. You can really see how someone like Harold Simmons is using his wealth to try to shape the 2012 election. There are quite a number of donors who have given in the $100,000s and some that have given many millions of dollars.
Of course, in a true democracy these people would still get to be heard. They’d be able to express an opinion any time they wanted and they’d be able to vote. Once. On election day. Same as you and me.