For what it's worth (prob not much) I used to work as a charity accountant (volunteering).
We had to pay for rent, rates etc. Then we have to pay for all the HR and safety and training and CRB checks that comes with volunteers and those paid little. We also had to pay someone to coordinate all those volunteers because it is tough to find someone willing to volunteer almost full time or take a cheap wage with the right skills and experience. So some of your charity money does go to big salaries.
However, you can always make a donation and have it written and bound that it can only be used for a particular area. I always write a legal document to have my donations specifically on research and not towards overheads and I let my sponsors know.
When I was at the charity we got sent a letter about London Marathon. We were asked if we'd like to handover 500 quid to pay for a place and we'd get a spot on their website blah blah blah. The more we paid, the more advertising we got. This is why London asks for high prices, to off-set what the charity have paid for the place. And the saddest thing is that if that runner tries their hardest and still falls short of the target, some charities will 'blacklist' them with the organisers and charge their credit cards what they have not managed'. That, I think is very wrong and I do wish London organisers would make some attempt to let more people ballot and choose their charity.
Rant over, la la la...