It's possible that GDQ staff (because I am sure they don't have enough to do!) could send out a post-marathon survey to a random sample of donation e-mail addresses that might include a checkbox for "Which of the following are reasons you donated (choose all that apply):" and include options like "I believed in the charity's cause", "I wanted to support the runner", "I wanted to say thank you for the marathon", "I really wanted a donation incentive met", "I wanted to win one of the prizes", etc. Then you could correlate that with specific donation incentives and/or prizes and really get a sense of what drives donations. On the other hand it is worth remembering that while raising large sums of money for awesome causes is fantastic, the difference in raising $N and $N+$200,000 is probably secondary to the goal of putting on a great speedrunning marathon. Well, that and staff time is limited and this kind of a survey project is not a quick thing to pull off.
Your Mainbar: