Mmmm, that's a pretty good question!
I didn't find anything on Lurker's Guide, but you could ask around in B5 forums like FisrtOnes.com or B5TV.com.
While waiting for an official or more educated answer, the only things that spring to my cloudy mind goes like this :
The value of Ducats would vary wildly depending on the year/event. For instance, a Ducat would be worth several EA credits in the years following First contact since the Humans were the primitives ones... But as EA gets bigger and stronger with the passage of time, while the Centauri Emperium is declining, the relative value would get more at the same level. And at the times when the Centauri were at war with... almost everybody else, you could bet that the Ducat's value plummeted accordingly.
On the top of my head, I would set the default value of a ducat at 5 credits in the first year of the show. After all, they seems to be pretty hefty golden coins.
Just my 2 ducats... :wink: Your exchange rate may vary.