Bruce,

$139 isn't too bad a deal, but what you may find is that you can get a cheaper price by buying a return ticket, with some notional date in the future for the return journey. Eurostar have an airline-stlye fare model, so the closer you get to the date of travel, the more expensive it's likely to be. I would imagine that the walk-up fare is quite a bit more than $139. I heard on the radio recently that they were doing very cheap deals for same-day returns (even though you'd only do the outbound) - somewhere around GB£50 ($70) but I know the availability for these was limited.

Hope this helps,
star_world