This is the cheapest that I can find for a My Passport 500GB at the moment. It's the updated model which I don't think has been officially released yet, hence the shipping delay from Amazon. Next best I can find for a My Passport 500GB is £69.99 for the old black version on Amazon and Ebuyer. The new 500GB is also available in red for £69.99 or blue for £89.99.
Update: it's now changed to out of stock, but you can still place an order - but there's no indication of how long you'll likely have to wait