You can "Prnt Scrn" from Marriott's own website and use their conversion calculator.

It is java, so I don't think you can enter a link. It is on the right whenever you go to date-specific reservations for a property.

Conversion:
100 GREAT BRITAIN POUND (GBP) = 189.39 UNITED STATES OF AMERICA DOLLAR (USD) as of Monday, April 18, 2005

Note: Converted amount is based on today's exchange rates and is for comparison purposes only.

It says "for comparison purposes only", but I think they would have a hard time arguing a different rate should be used.

Sam