If you are looking for cheap flights from the UK to the USA I have always found ebookers.com to be consistently the cheapest. I have been to Vegas a few times, though not for the Expo and I'm afraid to say that the flights are more expensive in September than other times. As far as cabs go you are looking at the same price in dollars as you would pay here pretty much. If you are heading out that way I would definitely advise going to the Grand Canyon and Hoover Dam if you can manage it.
If you are looking at staying at one of the bigger hotels a good website to see whats on and what they offer is www.cheapovegas.com
although I wouldn't book through that site it has a lot of info on it.
Hope this helps,