There is no question about it, flights are getting more expensive to Thailand. I remember when it used to be easy getting a sub £500 flight on the Etihad website without having to wait for a sale – now those kind of prices are for sales and when you do get notification, its a sub £500 flight to Abu Dabi!
The cheapest return flight I have found these days is Jet Airways I have got a flight with them leaving from Bangkok to London (always more expensive this way round) leaving on the 20th of December and coming back on 3rd Jan for £450. This was a sale price and now its more like £700 for those dates, but the point is, if you look for sale prices, you can still get sub £500 flights. I would try signing up for newsletters to a few airlines who fly to Thailand and look out for the discount prices.
I have seen really cheap flights leaving from Berlin. It is worth looking for flights from here, you just have to try and get a cheap flight to Germany – not too hard with all the discount airlines these days such as Ryan Air, Easyjet etc.