Does anyone know a way to get the currency exchange rates for paypal?

We have custom shopping cart and use Paypal (Website Payments Standard) to handle payments. Our 'home' currency is Euro, but we would like to present our customers the option to pay in different currencies (USD, CAD, AUD and GBP).

PayPal offers the option to:
    a) automatically convert our Euro quoted prices to, for example, USD upon checkout
    b) checkout in USD directly

With option a):

• We get paid in Euro, the customer pays for the currency exchange (good).
• The customer does not know what he/she is going to be charged in USD until checkout. (bad)

With option b)

• The customer pays in USD, then the currency is converted into EUR and we pay the the currency exchange.
• The customer never has to worry about the different currencies (excellent)
• We do not know the exchange rate PayPal is going to use so we cannot quote the correct prices to our customer (showstopper)

So my question is:
  Does anybody know a way to get the PayPal exchange rates?
or
  Does anybody know how to make a good estimate?

Update:
PayPal updates it's exchange rate 2 times a day. (at least, that is what they state). They use the Interbank Exchange Rate provided by ??? and add a 2.5% spread above this rate to determine their retail foreign exchange rates. Unforunately, there the Interbank Exchange Rates vary from source to source and from minute to minute.
We have been monitoring the PayPal exchange rates and cross referenced them with the Official reference rates provides by the European Central Bank. the results vary widely, somewhere from 1 to 6 ! percent...