The expression goes back to days when most middle-class people bought shoes hand-cobbled to their own measure. These were of course more expensive than today's mass-produced shoes, and it was routine to extend their life by periodically replacing the ‘shoe leather’ which suffered the most wear—the heels and half-soles.
This literal sense of ‘shoe leather’ came to have figurative senses.
‘Shoe leather’ reporting, for instance, is the style of journalistic investigation which involves ‘hitting the street’ to hunt down documents and sources rather than sitting in your office reading news releases and telephoning official spokespeople.
In economics, ‘shoe leather cost’ has the specific sense of the cost of the measures you undertake to overcome inflation:
One consequence of inflation is that it imposes "shoe-leather" costs on society. Essentially, shoe-leather costs refer to the time and effort people take to minimize the effect of inflation on the eroding purchasing power of money. People wear out their shoes on the way back and forth to the bank, so to speak, trying to protect the value of their assets.
Of course, the idea of wearing out your shoes implies more than literally walking to the bank more frequently than you otherwise would. It suggests that people spend their time and resources to manage their money and other financial assets, i.e. inflation-hedging activities, rather than using those resources to produce goods and services.
—"Looking at Shoe Leather Costs of Inflation", Michael R. Pakko, Senior economist,
Federal Reserve Bank of St. Louis]
The phrase at hand, keep you in shoe leather, means, literally, to provide you enough revenue to pay for maintaining your shoes—more generally, to cover the costs of keeping your business open and maintaining the value of your capital investment.