So $1 spent on healthy food instead of junk food would create the same amount of jobs, correct?
No. It will significantly reduce the efficiency, if not render it completely inefficient. By exerting control over a market such that demand for a specific product is artificially increased the demand for other commodities is inversely affected. This is a very complex process and I'm sure I'll risk sounding like a foil hat wearing Paulbot (I assure you, I'm not) I will say that meddling in the economy will have reverberations throughout. The inverse effect on other commodities will cause job losses in those markets. Overall, a net gain in jobs could still be found, but it will certainly be a smaller gain. The idea is basically that the gov't would be subsidizing only certain commodities. The only way to safely stimulate, is to stimulate aggregate demand. It is essential to diversify.
However, the mechanism can be used in other ways. For example, gov't subsidized corn is like an economic weapon against Mexico. You see, it was their cash crop, hell it is fucking sacred in the culture but ever since NAFTA, there have been fewer and fewer corn farmers in mexico, and now Mexico buys American corn, and everytime you buy a gallon of ethanol with corn in it, another Mexican has to come north to find work.
There is a bit of exaggeration and sarcasm in this, but it conveys the simple fact that nothing in the economy is a vacuum.
The very reason why foodstamps are such a good stimulus, is that they force the money to go into such a wide ranging sector of the economy but never to stray into narrow corners of it. If demand for junk food is reduced, the price of junk food will only fall while Doritos and Coca Cola lay people off. Conversely, the very products being chosen will rise in price, eating away at the efficiency of the program even more and affecting diets for everyone.
However, since companies like these use so much corn, it is possible that more ethanol could be made. So there is a possible good effect among the bad.
