I need to get out of London!

When? With enough lead time the train isn't so expensive - and don't you get a discount with the pass? Otherwise you should probably look at Eurolines - http://www.eurolines.com/ Have you checked budget airlines (not necessarily out of Heathrow - London has other airports) - try whichbudget.com.

allison, When will this trip be taking place? As the previous reply mentioned, using the budget airlines is one possibility. However, it will be important to book early in order to get the best prices. Which airport are you landing at? If you decide to use a budget airline, it will likely be departing from Luton, Stanstead or one of the other "peripheral airports". You'll need to allow enough time to get there from your arrival airport. Good luck!

As others said, it depends on how far out your trip is. It also helps to know what time of year you are traveling. Off-peak, shoulder season, and peak summer travel prices can vary a lot. Booking ahead and in the off-season, budget airlines can be dirt cheap. I booked an easyJet flight from London to Berlin for about 35 Euros last October.

My plane gets into Heathrow on June 21st. We plan to spend a few days there and then move on, hopefully to Paris.
It's such short notice, that's why I'm worried. We recieved some terrible misinformation about the Eurail and I was too naive to check other places! thank you all for your help, we need any advice we can get!

Eurolines bus is probably the cheapest. Way too late for decent fares on Eurostar. On Friday the 24th you could go from Victoria Coach Station at 10, 11, or 12 and arrive at 6, 7 or 9:15pm for as low as £28 one way per person if you book NOW. Curious minds want to know. What terrible misinformation about the Eurail did you receive? From where?

How about doing it the old school way and taking a train to Dover, then grabbing a ferry to Calais. From there you can put your Eurail pass to use. Below is the advice of the Man in seat 61: http://www.seat61.com/London-Paris-ferry.htm Money-saving suggestion:  Buy a 'day Trip' ticket from London to Calais, even for a one-way trip...  It's very poorly publicised (it's not even mentioned on their website!) but South Eastern Trains offer a day return day tripper ticket from London to Calais for £33.90 adult, £16.95 child, or £22.35 for Senior or Young person railcard holders (2011 prices).  It's valid on any train to Dover even in the morning peak and on any P&O ferry to Calais, no reservation necessary.  You can buy it at Charing Cross or Victoria stations on the day of travel.  It's intended for day trippers, but even for one-way trips this £33.90 train+ferry ticket is a lot cheaper than buying a £34 one-way train ticket to Dover plus a £30 foot passenger ferry fare to Calais.  There is no problem buying one of these day returns and not using the return leg.  Just throw the return half away!  Indeed, if you can buy a London-Calais day return ticket in advance in the UK for the date you want to come back, you could probably use the return leg of a second day trip ticket as your inbound one-way from Calais back to London.  The ticket can also be bought starting in many other stations in Kent & South London such as Croydon, Guildford, Woking, Orpington, Canterbury or Ashford.  It doesn't cover the bus between Dover Priory and Eastern Docks.  You can confirm prices & details by calling South Eastern's customer services on 0845 000 2222.

Your cheapest option would be the overnight Eurolines bus. A low fare plus you save the cost of a night in a hotel. Assuming your Eurail pass covers France, you could buy a passholder fare ticket (£58) on the Eurostar. Here's a quote from the Eurostar site: "Passholder fares cannot be booked online. They can only be booked at a Eurostar Terminal or through our contact centre on 08432 186 186 or +44 (0) 1233 617 575. All tickets are sold subject to availability." From the US dial 011 (US international access code) + 44 (UK country code) + 1233 617 575. Leave off the zero in parentheses. It's only used for calls originating in the UK.

....or how about "the only band that matters:" http://tinyurl.com/3v45289

Thank you so much for all your advice. We have booked a bus -to-ferry combination surprisingly cheaply.
You guys are fantastic!