The Millionaire Camp is a campsite for millionaires. You have to have over $1,000,000 but under $1,000,000,000 to be allowed in. Having over $1,000,000,000 means you go to the Billionaire Camp, next to the Millionaire Camp, separated by a fence.
When billionaires learnt Mr. Burns lost 4 million dollars short of a billion dollars, he was kicked out. Billionaires threw him over to the Millionaire Camp, having only $996,000,000.