Here's a URL that gives you only the number you're looking for. (That is, it doesn't give HTMl, just a number - No parsing required!)

http://finance.yahoo.com/d/quotes.csv?s=USDJPY=X&f=l1&e=.csv

Replace the six-letter string in uppercase with the currency symbols. You can play a little with the other parameters, too.
--
Jorge Gomez