Megabus currently sells bus tickets for only £1/€1 to go to Paris or Amsterdan from London. With booking fee of 50p you can get a return trip for £2.50!
London ->Paris will take 7hrs30 during the day and 9hrs30 during the night.
London -> Paris will take 9h45 during the day and 10hrs30 during the night.
For the London-Paris route, Megabus goes from Victoria stop and arrives at Paris-Porte Maillot, in the center of Paris. So no needs to get a shuttle and pay extra fee.
20kg of luguage is also included in the ticket price.