You asked the max I would be prepared to pay. Depends on how desperate I was to get there. When my sister and I booked last year we wanted direct flights but left it too late to book for half term so flew PE going and Upper Class coming back, it was more than £600+....On that occasion I was prepared to pay it ( not cos I'm rich but because it was important to travel at that time) but it wouldn't be a regular thing[msneek]
I think £500 for a direct flight in peak season is not unreasonable, but would hope to get one for a lot less off peak and booking well in advance.