There are two types of money in the game and they can't be used interchangeably. Both are named with puns: Sx and ProN. The amount you currently posses is displayed in the lower left corner of the menu screen. (The ProN counter appears only after a certain point in the game.)
Sx are crystals. They also come in many different concentrations with different values - so if the party is carrying 10,000 Sx, they're not carrying 10,000 different little crystals. In the Stineford magical library you can learn that this currency is generated naturally by monsters (which explains why monsters drop Sx when killed). In very large amounts it can be used to ward against monsters by concentrating it. The latter point is relevant to an optional quest at a certain point of the game. As Hilstara says:
|“||Many monsters are predators that target humans. Without Sx defenses, cities would be far more dangerous.||”|
Their power declines with time, as the Sx wear out once it has been refined. So as the group transport it, the crystals attract monsters, apparently.
So countries with huge amount of monsters are potentially rich and banks are (theoretically) not attacked by monsters.
Sx can be found in outer realms, as Robin explains during the visit to the Extradimensional Space that:
|“||It's an elemental material, so it's not unusual.||”|
"ProN" is short for "Promissory Notes" - bills of exchange created by the banks and used for large-scale financial transactions, such as business investments.
For game balance, you can't convert your Sx to ProN, and you can convert your ProN to freely usable Sx only at certain points of the plot (you are able to do it freely in the Bank of Stineford in Chapter 4). If it's possible, conversion can be done by talking to Megail or at a bank. The rate is (usually?) 1 ProN = 1 Sx, but you can only convert fixed amounts and in most cases you will convert with a loss.