I'm pretty sure that the vast bulk of money raised by the FNRttC Martlet's ride in September goes straight to the target charity.
In all likelihood, since a lot of the effort on the ride is done by regular FNRttCers, and Simon obviously does a lot of work on it for no cost.
For last years smaller Martetts Ride in March, their would have been some expense in terms of the lorry and minibus used as a sag wagon, but I believe those vehicles actually belonged to the Martletts, and I presume the people on board were volunteers. Possibly the main expense would have been petrol.
I presume that for the larger ride this year there are likely to be other expenses which will have to be paid for, like public liability insurance, although Simon has found some effectively free solutions to expenses. He's got NCP to allow him to use a London car park as an assembly point in exchange for some small amount of advertising.
I would feel uncomfortable doing a charity ride where the money that people donated was in effect partly being used to pay for my holiday.