Pedantic point: Technically, the multiplier idea you describe originates not with Keynes, but with one of Keynes students, Richard Kahn.
Substantive response. No question there is a multiplier effect, though the size of the multiplier has been much debated. Also much debated is whether the multiplier is bigger or smaller when the spending involved is "countercyclical" (i.e., done at the time of a "downturn" in order to stimulate the economy to increase).
No one disagrees with the multiplicative notion -- after all, apart from Scrooge, no one holds on to money forever. All of us either spend it today or save it and then spend it in some tomorrow.
But that is true of *all* spending, private as well as by the state. And that's the biggest problem with the Keynesian multiplier story that you have bought. Why do we think that the recipients of government spending are going to be more likely to immediately turn around and spend more themselves than current possessors or funds or than recipients of funds spent in the private sector?
After all, whether you fund the expenditure with current tax dollars or with future tax dollars through deficit financing, you're taking funds away from potential spending elsewhere in the system.
Unless you believe that the government is better at making spending decisions (i.e. at putting money where it gets re-spent more and better), the *net* multiplier could very well be less than one. That is, for every dollar of government expenditure, more private expenditure (through its multiplier) is lost than is gained through the multiplicative effect you describe.
To be sure, we have a correlation between the amount of government spending and the amount of the overall wealth increase. But correlation need not be causation; the real reason for the growth in wealth may not be because of the government expenditure but *despite* that expenditure's net drag on the system.
Take your example of $6000 of expenditure. If the multiplier turns out to be 0.75, then the net effect is $4,500 of new value at a cost of $6,000. Or a net of $1,500 lost and not able to be spent ANYWHERE in the economy. Another example is the idiocy called the "Cash for Clunkers" program, where the cost per car worth $20,000 or less ended up being $80,000 or more.
Historically, however, the private sector in this country has been so productive that they can get a return on investment that is not only positive, but remains positive even after having to make up the $1,500 and the $60,000 and the extra $5000 paid for DoD toilet seats and all the rest.
The amount of deficit financing by itself isn't the problem. The problem isn't even the amount of government spending overall. The problem is *what* the government spends the money on.
Keynes was a member of the small-but-highly-educated British elite. He believed in a big money multiplier because he believed, as did most of his fellow members and consultants to the British civil service/political leadership, that said civil servants (a/k/a government decision-makers) were better at choosing where to spend money than the part of the British population outside government. Just as today's elitist liberals and conservatives in the federal government believe that they are better at knowing where to spend other people's money than all of us are.
I think Keynes was wrong, and I think those modern feds are wrong.
Profoundly, horribly wrong.
America has been better at creating *new* wealth than any society anywhere and anywhen in human history. And Americans since World War II have been better at creating *new* wealth than Americans before and during World War II. But if America had not taken the approach to government spending that the second Roosevelt started us on and every administration and Congress since has worsened, we would have created even more wealth.
A lot more.
I'm often accused of being an elitist. But if I am, I am a far different kind of elitist than Keynes, today's federal politicians and bureaucrats, and today's Fortune 100 CEOs. I'm the kind of elitist that believes that ordinary Americans (those you describe as "good, normal Americans" and those who are out there marching to the beat of different and wacko drummers) systematically are better spenders that don't need anyone's help to spend their money in the most productive way.
That's what I believe.
And do not be conformed to this world, but be transformed by the renewing of your mind, that you may prove what is that good and acceptable and perfect will of God.
Romans 12:2 (NKJV)