It's virtually impossible for people to answer questions like that here; I would try expedia.co.uk and various other online travel firms, such as flightbookers and trailfinders. If you still have some of yesterday's papers, the travel section will have ads for these firms.
Subject to that, I'd be surprised if you'd get it for less than £500, not including tax, so you should expect to pay somewhere in the region of £550-600. KLM and Lufty have good deals from time to time; so does Alitalia, but there's a limit to everything. KLM would be my personal choice - AMS is the perfect hub and KLM has PTVs on its 777s.
Direct flights would be around £200 more expensive (although again, that's a generality.)