The profit maximizing firm will strive to find a mix of intputs so that the marginal rate of technical substitution equals the factor price ratio, or:

Notice that with a little simple algebra we can see that this profit maximizing behavior also means:

The Marginal Product of an input divided by its unit cost gives us the Marginal Product Per Dollar (or unit of currency). Naturally the firm will wish to obtain the same value from the last dollar spent on captial as the last dollar spent on labor. If one input generates more output per dollar the firm should use more of that input and less of the other.