Very hard question simply because it depends entirely when you are going and also prices vary considerably from direct to indirect flights.

I once paid less than £300.00 but it did involve going via Amsterdam to Detroit and finally on to Orlando with the trip taking nearly 23 hours and though it was fun at the time im not sure i would do it again unless it was very cheap and if you have kids its not even an option.

My upcoming trip next month i paid £450.00 with Virgin/Continental,though i did miss a better deal a couple of times by ummming and errring if you get my drift.

If i was going in July/August and flying direct with Virgin i would be willing to pay up to £600.00.

John.