How much are "cens" in FMA worth? Is there some sort of canon answer to this, or can something at least be inferred from the manga / either anime series?
Working off of some manga / Brotherhood details, we can reach some sort of approximation. I am not sure if there are any details in any of the guidebooks to give a clearer answer and so am only working off what I've seen in the series itself.
Ed borrows 520 cenz from Roy. He uses this to make a phone call (via a pay phone) to Winry. Admittedly this may be a costlier "long-distance" phone call, but this doesn't matter much - from later details we can still reach a similar conclusion.
When Roy asks for the money to be returned, Ed first gives him 500 cenz. Roy gets annoyed, and Ed tells him he's stingy for caring so much about 20 cenz. Ed also pays back Roy in coins.
In the Hong Kong dub of FMA, 520 cenz is translated to be $5.2, which isn't much in Hong Kong dollars.
According the Wiki (which argues that cenz is approximately equal to JPY), cenz comes both in paper bills and coins. This, along with the fact that we see Roy with a bunch of coins when he calls Hawkeye, implies that the coins should be of lesser value (which is usually the case in real life currencies that also use paper). After all, pay phone calls within the same city can't cost that much.
Given that Ed gets 520 cenz coins from Mustang (as seen at the end of this video), and taking the considerations in the last point into account, 520 is a relatively small value.
Some people have argued that Ed was mad about having to pay 100 cenz for coffee in Briggs, which might imply that he finds it too expensive. But given the above and the context (him being given a cup of coffee by a doctor, which he finds out afterwards that he's to pay for), it's more reasonable to assume that Ed is annoyed by the fact that he has to pay for (bad) coffee in the first place (without warning).
Consequently, the value of a single "cenz" doesn't seem to be much. Given that this is a Japanese manga, the most reasonably assumption would be that "cenz" is approximately equal to the Japanese yen in terms of value and/or worth.