Paris Expo.
Done some looking and asking about this as I'm set to go.
The Keynote and actual Expo are on two different sides of the main area of Paris. A good place to stay is bang on the middle near the Champs Elysees or that big symbol of 'Gallic power' the Eiffel Tower. There are literally dozens of hotels around this area and accomodation can be had from the sublime to the ridiculous (30Euros p/room per night *5,000 Euros per night - if your name is Steve Jobs).
Entry is a measly 12 Euros, and one-day return train tickets from London Waterloo are from £29 if yo book now. You need to register for both Expo AND Keynote, as the two are separate and just registering does not give you guaranteed access, let-alone a seat 😉.
Paris itself doesn't necessarily have to be an expensive place, though I'm going for the weekend with the other half, so it probably will be. 😱