I think a lot of the posters in this thread have not really touched on something that is really important, and a lot of you are really missing the point. First, What makes something valuable is its rarity, usefulness, desirability etc. A lot of the comments have touched on this but they look at it from more of a birds eye view. Keep in mind the average citizen of the inner sphere (even on a good planet) has a boring "earth like" life. They aren't MechWarrior's or interstellar adventurers or anyone you would want to read about. They are just normal people living their lives and making 10,000 CBills per year (on average). So ~$50k USD. Do they still want jewelry to wear? Yes. Do they still want the latest and greatest personal entertainment? Yes. Do they still want nice watches and things to impress their peers with? Yes. People are people and by and large in the BTU the majority of people live boring normal lives. Gold, silver etc. still has good value to these people just as it does to people today. A local lord or planetary noble is certainly going to place value on it, to impress others for no other reason. The spread out interstellar reaches of the BTU means due to simple supply / demand principles these highly differing worlds are going to have highly different values for the items that are scarce on their planet. Also the "interstellar economy" is significantly more limited than the "world economy" of today. Sure technology has progressed immensely but travel is limited to a level similar to the colonial era (with similar travel times to far flung realms of the crown). So unless you are on an inner world, as a typical person (participant in a world economy), you would never experience off world travel or in most cases off world goods. It simply is not cost effective to import things if they can be produced on the planet itself. This is going to make a huge value discrepancy between worlds as on one planet, water or equipment may be very limited and thusly expensive. On a high water percentage planet with a large population base, precious metals might indeed be very valuable, and water almost free.
Now we get into the concept of the "haves" and the "have nots". On a lot of planets that are not earth like and may not even be habitable they may have huge germanium deposits but no water. That means water would have to be literally created by smashing oxygen and hydrogen together in a reactor, extracted from something or shipped in from a nearby source. This all adds huge costs to just getting enough water to drink, bathe in, etc. So suddenly water becomes valuable, not so valuable you are going to necessarily use it as a reserve for your currency, but valuable nonetheless. So its not far fetched to think that some germanium miners in an asteroid belt might start to use water rations as a medium of exchange, and large quantities of water would be worth a significant amount as the nearest most cost effective water source might actually be in another star system. Just like someone else pointed out about the Gold Rush. The value of shovels, ammunition, fuel, rifles, etc was immense, and compared to the gold nuggets that people were literally just picking up in creek beds very expensive. However back East, a shovel was cheap and the gold nugget was ridiculously more valuable. These same factors would be at play when you are talking about the cost limitations of interstellar travel.