example_usage.rb

require 'money'
require 'xe_exchange'

Money.default_bank = XE::Exchange.instance

ten_dollars = Money.new(1000, 'USD')
ten_dollars_in_pounds = ten_dollars.exchange_to('GBP')

xe_exchange.rb

require 'open-uri'
require 'hpricot'
require 'singleton'

module XE
  class Exchange
    include Singleton
    
    def initialize
      @scraper = Scraper.new
      @rates = {}
    end
    
    def exchange(cents, from_currency, to_currency)
      (cents * get_rate(from_currency, to_currency)).floor
    end
    
    def get_rate(from_currency, to_currency)
      @rates[[from_currency, to_currency]] ||= 
        @scraper.get_rate(from_currency, to_currency)
    end
  end
  
  class Scraper
    URL = "http://www.xe.com/ucc/convert.cgi?Amount=0f&From=%s&To=%s"
    
    def get_rate(from, to)
      document = open(URL % [from, to]) { |f| Hpricot(f) }
      elements = (document / 'span.XEsmall')
      (elements)[1].inner_text.match(/(\d+\.\d+)/)[0].to_f
    end
  end
end