You can typically get a “good enough” read on your tests by mailing only 5,000 pieces of each one. My apologies to the statisticians who are wincing at that statement, but let’s face it — when money for acquisition is very tight, we make do. It’s not perfect, but it’s a place to start.
Someday you may have a budget to test a million pieces of donor-acquisition mail. But until that day comes, keep stretching those dollars and striving for constant improvements in your results.