30grams Gold = 100Cr
30 grams Silver = 10Cr
5 grams Silver = 1Cr
At that rate, I'll trade you all my 10cr coins for 10x as many 1cr coins, then melt those down and cast them into 10cr coins; I'll be rich, nearly doubling my money each time!
If you want to have a consistent monetary base, then be consistent. For instance, if 5 grams of silver is worth 1cr, then 50 grams - not 30 grams - is worth 10cr. Anything else, and people just melt the coins down and convert them to the other and you get rampant inflation.
Also, historically, gold has been worth about 50-100 times as much as silver. I don't know about copper, but it's probably not unreasonable to say it's 1/50-100th the value of silver. This gives you a good, useful rate.
1 copper = 1 cent
1 silver = 1 cred
1 gold = 100 cred
And you can have other denominations by making the coins proportionally larger or smaller.
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There needs to be SOME standard against which to compare things in an "ideal" environment. Sure, water is going to be worth LOTS more on a vacuum world than on water world, and indeed, you COULD make a fortune shipping it (so long as it's cheaper to go from star to star and not stay in the same star system), but overall, water has a faily low intrinsic value.
Once you know what something is "worth" in a "normal" environment, then you can apply situational modifiers. Some worlds may have a LOT of those, but that is where you find your trade routes.
Now as to the actual question of what a standard unit of exchange would be... historically, gold and silver have been used, because they had very few industrial uses, but looked very nice. Same for jewels. However, in a place where every day is a fight for survival, pretty objects aren't going to be of much value. You have to offer something important. It's going to be all barter. You're going to ask for stuff they consider junk and give them stuff they need to survive.
In a low-tech society, which doesn't value pretty baubels, you have one thing left to compare with: labor. How much effort does it take to make something? There of course will be modifications based on the object's utility, and the amount of skill required to make it (similar to tech level requirements).
But baubbles probably wouldn't hurt either; livening up the mood can be important too.