Over the course of most games, you will very often see the player gain 10 times, 20 times or even 100 times as much money as he/she used to for performing the exact same action, which means the relative value of everything in the game world which ties into the economy has to scale accordingly. Predictably, they very rarely do. Often, this is because developers balance economies around the bare minimum the player needs to do to get through the game. If I stick to the critical path in an RPG, for instance, I may be missing out on over half the available money in the game, especially when you consider the tendency for developers to use money as a generic reward suited to optional activities (side-quests, mini-games). So, while maybe the player who just wants to get through the game and see the ending will wind up having just enough money to get by, the player who puts a bit more time into the game will end up with so much she doesn't even know what to do with it. Thus, problem number one occurs: inflation.
Give out monetary rewards that are balanced with game progress. The easiest way to go about this is to simply play through the game as the player might, add up how much money is gained in the process, and then compare it to all the things the player is expected to buy, or can buy, accounting for a deviation of +/-%. This is fairly obvious, but it's quite surprising the number of games I've played where it seems the developers just did not do this.