When you are citing a statistic or another specific fact, a signal phrase is often not necessary. In most cases, readers will understand that the citation refers to the statistic or fact (not the whole paragraph).

In purely financial terms, the drugs cost more than $3 a day on average (Duenwald, 2004).

There is nothing wrong, however, with using a signal phrase.

Duenwald (2004) reported that the drugs cost more than $3 a day on average.