My coworker's two daughters (ages 17 and 21) are participating in a sponsored walk on Sunday night, to raise money for cancer research, and she's asked us if we'd like to sponsor them. I'm pretty broke, but I'd like to give something - she's lovely and her daughters (both of whom I've met) are sweet girls. I'm just not sure what amount to tell her! What's a decent amount to pledge without coming off cheap?? Anyone have any advice??