If I get multiple e-mails with different amounts, I e-mail back the other guys and tell them that I already have a guy offering me a better deal. Sometimes I help them out and trade e-mails of the other guys to them. Maybe between them they can work it out.
That's not really how the scam works. What happens is they try to get you to send money via Western Union. This money is supposedly to pay for fees. They just keep stringing you along with the promise of wealth any day now.