One of the cool things about roleplaying games is that you get to engage in competitive haggling over the price of pretty much everything you buy. In the modern developed world, the art of bargaining has become diluted or lost, and many of us feel the thrill of trying to negotiate a good deal on the rare occasions when we get to do it.
The bad thing about doing this sort of bartering in a make-believe world is that you can easily lose all perspective of the true relative value of the things you're trying to trade.
The other side effect is that everything becomes conceivable as a base unit of exchange.
Watto: I have the shield generator parts and two ship-mounted ion cannons.
Qui-Gon: How many gold pieces do I have?
GM: You spent your last Republic credits on those underwater breathing things.
Qui-Gon: Oh, right. I offer him a blaster in exchange.
Watto: One blaster for all that? Get real.
Qui-Gon: Okay... two then. But you have to throw in a ten-foot laser pole too.
Watto: Even one ion cannon is worth hundreds of blasters.
Qui-Gon: Jar Jar, did you see anyone with blasters on the way here?