I want to know more about the character in your example to provide a good option because you could elicit a similar feeling using a different type of word altogether. Part of me would like to see:
When people asked him where the money was going, he quipped, "a good cause".
even though that is more witty. I also like the following, which, depending on the full context, could give off a similar "terse but honest" vibe:
...he shrugged, " a good cause".
...he leveled, "a good cause".
...he maintained, "a good cause".
I could go on, and, while I typed this out, some others have posted good answers. I'm a fan of context and character to choose the best "said" replacement.