It all boils down to money eventually, but even within the institution, it may not have the obvious appearance of dollars.
For instance, I went into one institution and a fellow half my size—probably 90 pounds—was able to walk between a crowd of guys twice my size. They parted ways the way Moses parted the waters. He was the drug guy; he controlled influence in the institution. He had people outside who, when they got out, would provide favours—i.e. cash, retribution, or whatever.
Drugs are the currency. The service provided for the currency can be a number of things, such as respect or whatever. It's amazing how valuable those materials are inside the wall, probably more than outside, because of their limited supply.