$100 is good (for a round trip, it's about the cheapest fare) - but do check direct with eurostar.com if you haven't already (and also on lastminute.com for package deals). Pricing is exactly like airlines - you save loads by booking in advance, and it's all capacity controlled. Peak trains will be more expensive.

Personally, I don't rate Brussels that highly for a day trip - it takes longer to appreciate its true charm However, Ghent and Bruges are wonderful places to visit, and only a short hop on domestic trains from Brussels South.

The journey is about 2h from London.