The net cost of Keynesianism to the system is the taking/destroying capital from the more efficient private sector to distribute it through the much less efficient gov't sector. They try to mitigate (lie about) this net loss to the system by talking about the "multiplier factor". This "multiplier" is just a rehash of the justification for the broken windows fallacy.
When Reagan was in, the number they gave was .28 of a dollar actually makes it through the government for redistribution .. government spending. The rest is destroyed.