It's out by Windsor, so just West of Heathrow - No bus that I know of goes out there all the way from London, but there's an excellent train service (and if there was a bus it would probalby take ages!) - There are half hourly trains from Waterloo (take about 1 hr to "Windsor and Eton Riverside") ... alternatively trains from Paddington changing at Staines to "Windsor and Eton Central" ... short bus or cab ride from either station to Legoland (they're pretty much beside eachother)
£68 for the coach trip is very expensive if it's just for the coach (or does it include admission?)