How to travel to London at a reasonable price?

Hi! I should be grateful to get your advice on how to get to London at a reasonable price.

I have to make a trip there on 5 November, going there in the morning, returning in the evening.

Having checked the Eurostar website, I am surprised to note that the price is the one copied below (495 EUR, both ways, for me and my son).

Is there a better way to go there on that day?

Thanks in advance for your advice. Richard

COPY FROM THE EUROSTAR WEBSITE;

Brussels to London: 1 Adult, 1 Youth (12-25 only)Itinerary 1 of 1Outbound - Saturday 5th November 2011Search for another itinerary09:29 - Brussels Midi10:26 - London St Pancras01h57m
(duration)Standard
Non flexible170.00€Inbound - Saturday 5th November 201119:34 - London St Pancras22:33 - Brussels Midi01h59m
(duration)Standard
Non flexible325.00€Total price for all passengers (average price per person - 247.50€ Fare Breakdown)

495.00€