I had bought our bus tickets back in mid July. Very often there are some good discounts involved when you buy that far in advance, and that was definitely the case this time. We paid only £7.14 ($10.75 CAD, $8.15 USD) each for the four and a half hour ride with Megabus. And that included £2 each extra for reserved seating so that we were guaranteed to sit together.
Departure time was 11:00am from Victoria Coach Station in downtown London. We left Auntie Nice's house just after 9:30am and that gave us lots of time to get to the bus using train and underground.