But money is fungible, even among different currencies; think exchange rates.

What’s the value of a currency that can only be used to buy limited items?

"The black market for food stamps — where recipients sell the federal vouchers for cash — has grown by 30 percent in recent years, the Agriculture Department said in a just-released study. (...) In figures, sales of Supplemental Nutrition Assistance Program vouchers brought in $858 million in cash in the 2009-2011 period, compared to $330 million in the 2006-2008 time frame."

Somewhat lower because of transaction costs associated with barter and black markets but it still has value.