I am due to arrive in London just a few days prior to the closing ceremony for the Olympics. The prices I have found so far are almost doube during that time. It seems reaonable afterwards even though the Para-Olympics follow, the prices seem back to usual after 12 August 2012. For example 1 night accomodation during the Olympics is the same for two night post Olympics and transfer costs are almost triple. I am assuming the food prices will escalate too. In any case I am thankful that my three month trip only includes a few days while the Olympics are on simply because I will be basing myself in London with the couple of tours I am going on departing from there.
Hope this helps. Have a great time.
Britain and Ireland 12 August - 25 August
Mega European 28 August - 15 October