With a game like Project Eternity it's important to strike a balance adding realistic details that give the world depth against any burdens those complicated details could lay on the shoulders of the player.
I feel that coins and treasure that encumber the player are an important part of old school AD&D and Basic D&D roleplaying experience, and that they add a sense of depth and believability to a world.
Your first concern with weighty gold might be....how on [whatever planet Project Eternity takes place on] will my party be able to amass any sort of wealth.
In Europe's early modern period (circa 1500), as well as in both the present and classical antiquity the purchasing power of a 22 carat gold coin the size of a dime (which would weigh about as much as a nickel) would be greater than 100 United States dollars today. When the exchange in New York closed last night the gold in such a coin would have been worth $263.81. With 200 nickels to the kilogram, that means a 5 kilogram (11 pound) sack of gold coins would have been worth over $100,000...likely more wealth than even some nobles in Europe's middle ages ever possessed. Multiply that by six party members and you start to see that your party could tote around a very reasonable amount of wealth.
As a side note a high quality diamond or emerald weighing even one gram (less than half the weight of a penny) would be worth an extraordinary amount then or or now.
Your next question might be, how am I supposed to buy arrows and bandages with gold coins that are worth that much. The despite the stereotype advanced in most roleplaying games the most popular metal for coins in the middles ages was not gold, but silver. Most nations were on the silver standard until the discovery of large amounts of silver in the Americas brought about well founded fears of inflation. Coins in the United States were made with 90% silver through the 1950s. In Europe during the middle ages silver was typically traded at between 1/12th and 1/16th the value of gold (in modern times it is worth much less). In Japan silver was traded at 1/5th the value of gold.
I would prefer the Obsidian include at least gold and silver coins in Project Eternity, and that they offer the option (even if it is only available in Expert mode) to play with coins that have weight.
