This is usually easy, as they're pretty cheap. We can get tens of thousands of impressions for several hundred dollars. But don't go purchasing such a small lot to be run all at one time. This will skew your test. If you buy only 50K impressions, and they run over the course of 10 minutes on a Tuesday night at 3 a.m., you could get some pretty funky performance results.
Controls are problematic, particularly with unique types of media or new types of media.
You need to purchase an awful lot of media to make sure you get a large-enough sample in any one geographical location. One last factor worth bringing up remains the issue of burnout, especially in vertical industries and the B2B space. Two, in particular, are traditionally B2B clients who are making their first consumer efforts. We know that their audience spends time online and can identify the sites that make sense. In this particular case, working with a narrow audience within the constraints of a narrow topic, it's quite easy to purchase too much media.