Wealth
Experience points (XP) are gained from quest completion, world-building, and another two sources: treasure and monsters. However, XP from treasure is only gained if it is spent. Thus, if a character brings 1,000 gold pieces worth of treasure out of a dungeon, he only gains one XP for every gold piece spent. This gold can be spent on anything, whether vital or frivolous, but it must be exchanged for something in order to earn the experience points.
The design intent of this house rule is twofold. First, it keeps player characters cash poor and thus provides them with ample incentive to continue adventuring. Second, it forces characters to use their wealth rather than simply hoard it. Some will fritter it away on meaningless pleasures and trifles, while others will invest it into large projects, such as spell research or the construction of a stronghold.
When treasure is spent properly to earn experience, a single gold piece (a royal) — or gold piece worth of objects d'Art, etc. — translates to 1.0 XP. Therefore, chroma translate to 0.1 XP, and shiny cuprous to 0.001 XP. PCs should normally trade in red-, black-, and aquamarine cuprous for gold before spending treasure.
Be sure to track treasure and wealth separately. Treasure represents valuables, including maps and information, which characters risk their lives to bring back to civilization. Wealth is acquired without danger. Only treasure generates XP when spent, which may also generate wealth. For example, characters who recover 20,000 gp from the dungeon record it as treasure and earn XP when they spend it on the deed to a gold mine. Gold mined from their new acquisition will be recorded as wealth, which can be spent like any other gold but will not earn XP in the process.
It is fine for players to try to spend their treasure in ways that will make them richer and better able to seek treasure. Although treasure must be exchanged for something, this may be abstract. A party which bribes an intelligent monster for information, or to be allowed to pass unmolested, certainly gains something if the attempt is successful. If the bribe is subsequently regained by killing or robbing the monster, it should be recorded as wealth rather than treasure, and may earn the party a reputation that hinders future negotiations.
The unambitious will be content to gather wealth, but those seeking advancement must be willing to risk their lives for treasure. Once gained, spending it before someone else takes it away becomes an urgent priority.
One of the most common ways to spend treasure is to offer it as a reward in exchange for the completion of some task which can generate wealth or improve one's chances of survival.