Monetary systems in Firefly are as well detailed as any technical infomation we're shown.
Yes, it's that bad.
Still, there are two good examples of the use of money in Firefly -- one example (in Ariel) shows us a rough conversion rate between the two main types of currency (the Core World 'credits' and border world 'platinum'), while the other (in Safe) gives us a rough idea of how either type of currency would convert into U.S. dollars today.
Example One - establishing conversion ratios:
In Ariel, Simon's explanation of the value of various medicines on the black market is given in both credits and platinum (at least for the first bit of medicine, Ivoprovalin). From this side-by-side comparison, we get a general breakdown that shows that one credit is the equivilant of 2.5 platinum. (At least on the black market, but that might be the only time it comes up: using Fray as an example -- another of Joss' creations taking place in a futuristic setting -- we can assume that using 'hard' currency on a core world is probably a crime in itself.)
(However, bear in mind that Simon quotes Ivoprovalin at 'fifty platinum, maybe twenty credits,' suggesting that the latter value is more than the former. Though any number of values could fit in this conversion, a plausible one is 1 credit = 4.8 platinum, presuming that one "platinum" is a grain of platinum. Why? Because then 100 credits would equal 1 troy ounce of platinum, the most commonly traded unit of platinum in the real world. This would mean that 50 platinum would equal 10 credits and change, and that 20 credits would equal 96 platinum. Such a huge range of value is entirely plausible for the street value of a drug. Anyhow, just a thought ...)
Example Two - establishing real world value:
In Safe, we see Mal arranging for the sale of cattle that are apparently too old to bear offspring and useful only as food. The original deal was 30 platinum per animal. The buyer complains that the animals are atrophied and worth less, offering only 20 platinum per animal. The two men finally settle on a middle ground of 25 platinum per head.
This might not be much to go on, but luckily, I come from an agricultural background -- my family actually used to buy older animals and fatten them up during the winter months when we weren't otherwise occupied -- selling them for a profit after they were fattened up and more appealing for those looking for food animals. We would buy them on the low-end for about 200 dollars per head (usually what we paid was a specific number of cents per pound, but it worked out to 200 a head) and aimed to sell them for about 300 dollars per head. Based on that, the conversion rate is pretty easy to figure: 20-30 platinum/animal vs. 200-300 dollars/animal mean 1 platinum = 10 dollars.
However, comparing the price of a cow on Earth-that-was over 600 years age to a cow on a little outer-rim moon in the 26th century might not be as accurate an estimation of the relative values of platinum/USD as we might like.
We must consider that the demand for cattle on an outer-rim world probably far outstrips the supply (otherwise the out-worlders wouldn't bother bringing them in from off world). This means that even if cows are an abundant resource on other planets (such as more succesful moon colonies), the price of a cow will also factor in the cost of fuel for interplanetary space travel. If we can assume that the cost of traveling between two moons is a great deal higher than the cost of trucking it between two counties (even during an oil crisis), then we can also assume that cows would cost a similarly greater amount.
As a note: it has repeatedly been established that real food is a commodity. While protein enriched food supplements might last a family for a month (longer if they don't like their children much), they taste like gorram mudder's milk. In a 'verse that supports molecular nanotechnology, it can be safely assumed that food supplements are cheaper and easier to produce than, say, cows. For this reason I wonder: why is there trade in barren bovine at all? I assume this is because the colonists are either unable or unwilling to contact the Alliance or the Blue Sun group for a shipment. If the out-worlders are unable, then that puts them in a desperate situation (thus higher demand). If they are unwilling it is because they are a succesful collony that is able to pick and choose what they eat, which means they are probably paying a higher premium.
(It is also possible that "real" food is considered a luxury, and therefore much more expensive. It actually makes more sense to say that the poorer Colonists eat just the raw mass-produced "protein." Even if there were trade barriers in place, smuggling protein should not, in principle, be a problem. Considering also that it's not easily perishable.)
The cost of high demand piled on top of fuel costs change the value of a cow enough that any estimation of its value relative to an American cow is probably wildly inaccurate. Therefore I suggest the following conversion be taken with a grain of salt:
1 credit = 2.5 platinum = 25 USD.
(Incidently, that would mean that the bounty on River that Jubal Early mentions in Objects In Space (200,000 credits) would be 500,000 platinum, or 5 million dollars. Blue Sun wants her back pretty bad, I'd say.)
(Going on the 1 credit = 4.8 platinum theory from above, the price of 1 troy ounce of platinum, as of October 1st 2005, is $950. That would mean 1 credit = 4.8 platinum = 9.5 USD. This would mean that the cows in the above example, however, are going at $50 a head - ludicrously cheap, especially for a commodity so dear that it needs to be lugged between planets. Therefore, the value of 25 USD per credit is probably closer to the 'real world' value)
For RPG gaming purposes, all you have to do to figure out how much something would cost in the Firefly 'verse is find out how much it would cost today and divide by ten... that's the cost in platinum.