well, I spend all my time coding currencies, conversions, etc though I do it in php but the logic is the same.
I manage it all in a database, rough structure would be
countries to currencies connection
>> set a session variable equal to the exchange rate and reset it every week
all well and good but you need to store it on your site somewhere and database is usually easiest. Remember you may need rates both ways as well.
what might make more sense is to tag what country people are from and then have currencies attributed to each country. Store the country in the session and then when you display prices you can display them in every currency attributed to that country.
I don't know if you want to drag conversion rates around all the time in the session, maybe, but that would be a decision you could make somewhere late in the design phase (and I dont mean the coding phase ;) )
another thing to keep in mind. If you are updating rates you will need to store the old rates, otherwise you may have some issues with how much things cost last month if you try to use today's rate against a purchase from last month.
ah, accounting, isn't it fun, talk about redundant data
that's more or less the rough outline off the top of my head