Nothing's free, but what you have to exchange for it will vary depending on where you are and who you're bartering with. See also: Where to Get the Best Stuff.
“Energy credits are one of the universe’s most common units of exchange, legal tender almost...Energy Credit
“Quargian Plenocredits are a form of currency. The exchange rate currently makes them extremely...Quargian Plenocredit
“Gilpots are a form of currency used on planet Klibbertot, planet Smoo, and in the Farflux Dimension.Gilpot
“Galactars are a form of currency issued by the Federation League of Planets in competition with...Galactar