Boring way: add an extra field to your Item class called priceUSD, and populate it with an overriden save method. means you don't have the burden of running calculations on every single query - just on every single update. so whether that's good or not will depend on whether you tend to write more or read more (given your question, maybe it's read?)
something like this:
class Item(models.Model):
priceRT = models.DecimalField(max_digits=15, decimal_places=2, default=0)
cur = models.ForeignKey(Currency)
priceUSD = models.DecimalField(max_digits=15, decimal_places=2, default=0)
def save(self,*args,**kwargs)
self.priceUSD = self.priceRT * self.cur.rateToUSD
super(Model,self).save(*args,**kwargs)
In my django stuff, whenever I've tried to implement clever calculated fields without storing them in the database, I've usually found it comes with too many disadvantages to be usefult (eg can't query on them, can't sort on them). so storing a proper field in the DB with a custom save method is how I've usually done it, and it works ok. you'll want a bit more error-checking and stuff tho.
