Murty Rabbitt

Hi there, wondering about when is a good time to book for flights DUB - BKK for the Christmas period. I know it's very early in the year so many airlines won't have flights on promotion, but is there anyway of knowing how much on average to expect they would cost? I can see from the Hopper app it says approx �770 and to wait until the price drops, as at the moment the flights are advertised circa �850. I have in my head paying �650 a number of years ago but maybe the prices have increased in the meantime?
I check regularly on Skyscanner but unfortunately the prices of some of the travel agents change once you click on their price and it brings you to their website.
When do Xmas promotional flights normally go on offer or any advice on what price is a good price?