I'd imagine that it bounces around a fair bit. The Federation's fortunes rise and fall, and and so does the value of the credit against foreign money. New resources are discovered, a spacial phenomena destroys a sector, pirates raid a trading corridor, members join and members leave again. Plus the Federation apparently can't stay out of wars.
I could see the Ferengi manipulating the value of GPL to keep it artificially low, they're the galaxies traders, they would want the people to come to them for their good and services, and not their local markets. The Ferengi might dream of the big deal, but make the majority of their GPL by undercutting the competition by a few percentage points.
The game source book for Star Trek Universe puts the exchange rate at one federation credit to ten slips of GPL. However, Star Trek: Deep Space Nine game rule book says one credit to one slip. I personal put the TOS era credit at five dollars in value, base on The Trouble with Tribbles initial bar fine for a tribble (10cr), verses the cost of getting my kitten out of the shelter (50 $us).