Well, in Action Stations, Ensign Tolwyn mentions that five hundred credits is "two months' pay for a newly-minted Academy graduate", and in the WC4 novel, Blair gives the guy in the bar five credits to buy dinner. Using that as a guide, I would estimate that a Confederation Credit circa the Kilrathi War period is worth between one and two 2002 United States Dollars.
As for fighters being cheaper in the 27th century, it probably has a lot to do with the factories being automated. We see in WC4 that the assembly line where they are making the Bearcats is almost totally automated.