This reminds me some discussion we had in my current gaming group when playing AD&D and trying to get an equivalent on what a gold coin (or silver, copper, etc) is worth in current currency. The bottom line was that it was almost impossible to establish a direct equivalence, since the relative value of goods can change a lot over time depending on the technology, the culture, and various other parameters. I expect for instance that, in Trek, a lot of manufactured goods (as opposed to replicated) would get the same increase in price as that we witness nowadays for hand-made (as opposed to manufactured).
I also always assumed that some of the Federation most powerful computers were used to keep track of the change rate with currencies from other powers and establish the value of the credit depending on the overall wealth of the Federation.
"The main difference between Trekkies and Manchester United fans is that Trekkies never trashed a train carriage. So why are the Trekkies the social outcasts?"
Terry Pratchett