I was lucky enough to get a place in the ballot for the 100mile RideLondon event this year, this is a closed road sportive following a similar route to the 2012 London Olympic cycle route. Around 30,000 people can take part with both a 100mile and 46 mile route on offer this year. A ballot is run to get places and these are randomly allocated, it is also possible to enter through a charity if you agree to raise a minimum amount. I assume that the charities have to pay a lot for these places but this seems like a similar method to what they use for the marathon events too. This event is actually organised by the same company who run the London Marathon so they have a lot of experience in big events.