We have to resort to other means. One way of counting is to use the number of activations. Unfortunately, Apple doesn't report those for tablets, and Google doesn't count most of the cheap ones, nor do they break out tablets from phones (except for reported Honeycomb tablets, which are undoubtedly the minority of tablet sales).
IPads are very popular (I just gave one to a daughter yesterday, even though she has an Android phone), but not everyone can afford them. So a lot of people are buying under-$200 Android tablets (which are usually best used as kids' tablets, but are okay for browsing and a few games. These tablets are usually not reported in Google's activation numbers.)
I am curious. How does Google know a tablet is under-$200 Android tablets?