(MONEY Magazine) – If you're a channel surfer, you've probably recently clicked into a 30-minute charity infomercial featuring a celebrity on cable networks or your local broadcast stations. These commercials tug at your heartstrings with scenes of children starving in Africa or coping with physical deformities. But grab a hanky, not your checkbook. The productions can be a mighty expensive way to seek contributions. A 30- minute appeal typically costs $100,000 to $250,000 to produce and $30,000 for test-marketing in selected cities. This high cost is one reason you are often pitched to make regular, monthly donations. ''The larger the lifetime contribution of a donor, the more you can spend to acquire him or her,'' says William Dixon, development director of the Christian Children's Fund. Reserve your donations for outfits that spend their dollars wisely. One often-used yardstick: the percentage of the group's income that goes toward good works. A reasonable benchmark would be around 70% or more. So before ponying, ask the group to document its percentage. By the way, among the charities airing infomercials recently, these three exceeded the 70% threshold last year: National Easter Seal Society (79.7%; 312-726-6200) and two relief groups, Christian Children's Fund (80.2%; 800-776-6767) and World Vision (77.7%; 818-357-7979).