... whereas I was a little surprised it's not higher: there are a lot of people on Google adwords paying more than £1 for every click.

I'm not sure if Google Grants may be relevant? Google has a program where the US Google gives registered charities a 'grant' of $330 per day, to be spend on Google advertising. In theory I could be 'spending' £78211pa on ads, just for one charity, but of course it's not real money. Well, unless a lot more people click on 'sponsor a dog' it's not anyway.