There are approximately 244,673,000 adults aged 18+ in the US: http://quickfacts.census.gov/qfd/states/00000.html 318,857,056 * (1 - 0.233% of Persons under 18 years, percent, 2013) ~= 244,563,362
And according to https://www.opensecrets.org/lobby/ , to match the $3.24 Billion spent on lobbying in 2014 you'd have to get absolutely everyone in the US 18 or over to give you 13$ and a quarter just to match what corporations are spending.
It's not the least bit realistic, and if the pre-2008 growth rate in lobbying spend picks up again it'll just get more and more out of control.
What's really necessary to get this absolutely incredible amount of money out of politics. The two presidential candidates shouldn't have to spend over 30 billion dollars on an election. The amount of money in politics right now is just insane.