i reckon it would be fairer if the tickets were expensive, and limit everyone to a maximum of 1 ticket, then the charity would receive maximum benefit, and the guys who don't have a lot of spare doh wouldn't be gazumped by someone who has stacks
say 50 people bought one at £20 per ticket, then there would be a grand for charity, which in fairness is far more than the bike is probably worth so one person gets a cheap bike, several worthy causes benefit.
as others have said its completely up to the OP but he wants to do something generous and this method would allow him to give on multiple fronts, the lucky winner, the forum and his chosen charity!
I could be wrong, but I think it unlikely you'd get 50 people buying a raffle ticket for £20 each, I don't know if you'd get 200 people to pay £5 for a raffle ticket, but I reckon that's more likely than getting 50 to pay £20 for a ticket. Rather than sell raffle tickets, how about selling something people might want, like a spoke card, could cut a corner off of each card to go into the draw. Not suggesting a spoke card is worth anything like £5, but given the choice between being left with a losing raffle ticket or a spoke card, know which I'd choose :)
I could be wrong, but I think it unlikely you'd get 50 people buying a raffle ticket for £20 each, I don't know if you'd get 200 people to pay £5 for a raffle ticket, but I reckon that's more likely than getting 50 to pay £20 for a ticket. Rather than sell raffle tickets, how about selling something people might want, like a spoke card, could cut a corner off of each card to go into the draw. Not suggesting a spoke card is worth anything like £5, but given the choice between being left with a losing raffle ticket or a spoke card, know which I'd choose :)