Question special

As a medical student, I carried around a $4 drug book with me so that I had an idea of the least expensive drugs for patients. Is there an online source for house staff to use when making decisions about which medications to prescribe in clinic or post-hospitalization? Obviously, this applies only when drugs are of similar efficacy (i.e. bactrim vs. macrobid for UTI, different PPIs for GERD, etc).