The weird thing about Eurostar vs. air travel is that going by air is sometimes cheaper and always slower—exactly the opposite of what you'd expect.

Eurostar is three hours door-to-door (city center to city center). Travelling by air is never less than 4-6 hours, door-to-door.

With Eurostar, you still have the transport to/from Waterloo and to/from Gare du Nord from your start and end points.

Not quite as long as getting to the airports usually (although getting to Heathrow can be very quick), but not something to discount in the total calculation.

You can do better than the $300 RT price for two that you got from Eurorailways. When I arbitrarily entered a departure date of July 16 (Sunday) and a return date of July 20 on eurostar.com, the outbound leg was listed at $75. The inbound leg was $45 for departures between 11:00 and 15:00 and $60 for any other time of day. So your RT price would be either $120 or $135.

Having just been, and looking through the fares, you really need to pick and choose from "off-hours" and "off-days" - Wednesday at 12:30 worked well. It is entirely possible to pay $500 if traveling at peak days/hours on exchangeable/refundable tickets.

When Eurostar fares are high, such as during the two-week Easter period when all cheap fares seem nonexistant as Eurostar is apparently trying to sell at higher prices you still may be able to get a passholder fare - available with any pass valid in UK, France or Belgium - these cost $75 each way and at times can be booked even when all the other cheap fares cannot. Thus if you buy a LondonPlus pass, good for two days of travel in an 8-day period in Southeast UK - about a 100 mile radius around London for $70, this qualifies you for the passholder fare - so this is an angle to be used if you find only expensive fares for the date you want to travel. and you may well be able to use the London Plus pass - day trip to Stonehenge, Oxford, Cambridge, etc., or to Gatwick Airport (but not valid on Heathrow Express). so if good fares are exhausted ask about booking passholder fare and buy London pass (see BETS above for great service)- even if you throw the pass away it may be cheaper than paying full fare at peak times on Eurostar. for example the two weeks around Easter there seem to be no cheap Eurostar tickets at all but my sources say passholder fares can at times be booked then. The 1st class passholder fare of $135 also can be booked easier than the $75 standard class passholder fare. It seems Eurostar is blocking out cheap tickets in hopes of selling at higher rates during these two heavy trafficked weeks - at times cheaper seats may reappear as the train time gets closer if it appears Eurostar won't fill them with more expensive tickets.
But to be guaranteed cheaper Eurostar fares you should book far ahead to be sure - especially for $45 youth fares, $90 senior fares and $90 round trips, $94 day returns, etc. Canbook nine months in advance in U.S. - even though train times aren't loaded and could change you'd be guaranteed a seat.