Perhaps it is not that straightforward. When I have made bookings in ET I receive a 'special upgrade' offer that come to less than booking CE directly. The thing is you do not know in advance what the upgrade price will be. Yesterday I was offered an upgrade to CE for £49 each way after booking in ET. If I had booked CE directly, it was an extra £88 each way. However, all my ET bookings recently, have received these upgrade offers, for between £49 and £69, all of which are less than the price difference between CE and ET at the time of booking.