Any help is appreciated: I am looking (so far unsuccessfully) for a number that nicely illustrates that people prefer their donations to help locally rather than internationally. 

P.S.: One number I found was TLYCS's claim that only 10% of individual donations go to international causes. However, this seems misleading. This claim is based on the Giving USA's report which has international affairs as one cause area. However, the alternative cause areas are not necessarily national since they include, for example, giving to foundations (which comprises international development as well).