I checked Transavia.com I see that they don't have an English version. Where is Roterdam airport? Is it the same as Schipol?
Also what is the tax like?
Try ba.com. You should be able to get a flight for about ��35.
Roterdam airport is not the same as Schipol.
I am doing the reverse next month - flying from London to Amsterdam. The cheapest fare I found was with EasyJet but with all of the taxes, exchanges rates, extra $5 for using a credit card, it came to almost $85 for the two of us. The negatives I saw about EasyJet was that they use the more remote airports and they have fewer flights. I ended up booking a flight with BMI out of Heathrow. The total price for the two of us was just inder $140. It cost a little more but will be cheaper getting to Heathrow and much better choice of flights - especially useful if there are any problems with the flight I booked.
Rotterdam airport and Amsterdam Airport are two different airports. If you plan to fly than make sure you're flying to Schiphol. The best alternative to flying is taking the train-ferry-train connection Londow-Harwich-Hoek van Holland-Amsterdam. If you take the night ferry and book a cabin, you safe out 1 nights hotel aswell and make full use of your daytime
I'd recommend EasyJet from London Gatwick (LGW) over BMI or any other airline from LHR. LGW is about half an hour from Victoria by train and generally cheaper to get to than LHR (the Heathrow express costs GBP 14 single and the alternative Tube to LHR which is cheaper takes forever - around an hour last time I did it, and I think there's engineering works until 2006 at Terminal 4 and the Piccadilly Tube station?).
The London-Harwich-Hoek van Holland-Amsterdam route might be cheaper, and in my experience runs fine (subject to weather in winter, when the high-speed boat may get cancelled in bad weather!) but it does really take a whole day. Overnight is OK I guess, but that makes it more expensive unless you want to sleep in a chair! You also have to think of eating on the boat etc if you are going to be travelling a whole day, and that's not up to much or is pricey (and not up to much!).
If you fly - when you get here take the train from Schiphol to Amsterdam Centraal - right in the center of the city - a single is Euro 3.40 and it should take twenty minutes max. Many of the budget hotels/hostels are walking distance from Centraal Station. A taxi - not recommended - would set you back about 30-40 Euro depending where in Amsterdam you want to be - take a TCA or Schiphol taxi, not one of the guys who might pester you in the arrivals hall.
Enjoy your visit.
Last summer Eurolines bus was EUR 48. If you could prepuchase 30 days in advance a return ticket could have been as low as EUR 39.