If you were to use a real world exchange rate, I think you would have to alter it for each game, based on how similar items are priced. FF inflation, I suppose. It would also be useful to attempt to compare the prices of commodities that have real world equivalents (i.e. inn costs, etc.). Ultimately, though, I don't think an effective conversion can be reached.

Why monsters have gil? I would imagine there's no real reason. It's probably just the simplest way for the player to acquire gil based on game mechanics.