Trade rate isn't based on how steady the money is. The value of currency is based on several things, but most of all it's based on how much money is in circulation. If the Gartagens have a higher exchange rate than the Nepleslians that just means that the Nepleslians print more money on a yearly basis than the Gartagens, and if there is more money out there that means it's going to be worth less.
Other than that exchange rate can also be based on economy. Theoretically, since the Nepleslians have been having all of the trouble in Funky Town the value of their currency will be lower because it's less reliable at the moment. However, the Gartagens are having no economic troubles and thus there is no reason for the dollar to be lower. However, this rule doesn't really apply in SARP since exchange rates don't constantly change.
Bottom line is exchange rates are just an arbitrary number that the FM made up and this number has no indication of being better, all it means is that one unit of a certain faction's currency is worth more than one unit of another faction's currency, which isn't necessarily based on which faction is stronger or older.
Example: The current exchange rate for 1 US Dollar is 81 Japanese Yen (It was about 90 a year and a half ago). However, the exchange rate for 1 US Dollar is .69 Euros. Thus Euros have a higher value than US Dollars, which have a much higher value than Japanese Yen. All this means is that the Japanese have much more Yen in circulation than the United States does Dollars, who has more money in circulation than Europe does.