Cost effectiveness has to do with more than just overall cost - it's also about the added benefits you get with costs. You could spend $100 for something of no value (zero cost-effectiveness) or $1000 for something that gives you a lot of added value/benefits. In this case, the study is showing that grade-level effects are bigger than individual effects in determining an individual's marijuana usage. So to get the biggest bang for your buck, so to speak, you would survey at the grade level.