In my experience, at least if one books tickets only several days in advance, the train is not much more expensive, but often comparably. It might be different if you book several weeks in advance, though.

For example I just looked up the fares for a weekend trip Brussel - London on the coming weekend. Leaving after work on Friday, getting back Sunday evening.

  Eurostar: Brussel to London: 17:56 19:03
            London to Brussel: 19:34 22:38
            233€

  Cheapast Flight (Brussels Airlines): 16:55 17:05
                                       17:50 20:00
                                       196€