The common currency of Ayrede, Aren are small iron coins. We don't find out the exact value of one Aren, but Captain Horatio Nebevay tips April an Aren when she delivers a map for him. As an Aren is made of iron, one could be forgiven for deriving its origin from this latter Germanic word. Its similarity with the word Ayrede no doubt also worked in its favour.
When April tries to play the cups game with her gold ring as payment, the Cups Handler tells her that he doesn't accept gold as payment. Iron, however, is a valuable commodity in Arcadia. This may explain why Arcadia's technological development is so limited.
In much the same way that April finds her Screwdriver, Calculator and Organic Plastic Leaf handy in Arcadia, a spare Aren she has in her pocket comes in handy on the Morning Star when she needs to unscrew an air duct. Unlike the latter two items, however, its use in Stark exhibits no unusual properties.
Under Azadi occupation the Aren appears to no longer be in use. When Brian speaks to Benrime he mentions that he can pay his bar tab as he recently came into some "gold". Perhaps, as the Azadi obviously have plenty of iron to make their weapons and machines, iron is no longer so valuable. It is also possible that they have simply imposed their own currency on the Ayredans.