It's a cop-out answer of a question but it's relative to the customer. convenience, travel time are all factors.

Most of my travel is for work I am always looking to save (and at the moment pressure from others) money on my travel budget. So If i find a EX Eu- out of AMS that is half the price of Ex LHR then that is close enough for me to save my employers money and still get the travel perks I negotiated into my employment contact!

However I would not go to CAI or BUD to save £500 or whatever figure because they just isn't convenient for me. Ultimately it's all in the beholder.

taking my awkward answer aside. I believe a J class return ticket that is £2000 or less and the flight time is over 7 hours is a good price.