OBJECTIVES: To calculate the percent change in the average wholesale price (AWP) of drugs over 1 year, and to assess the effect of drug price updates on the results of a standard pharmacy budget impact model (BIM). METHODS: AWPs were pulled from Red Book Online for the top 50 branded drugs with the highest sales in 2017 according to Datamonitor. Unit drug costs from 1/1/2017 were compared to 1/1/2018 to calculate an average percent change in price. A simple, 1-year pharmacy BIM was constructed from the US payer perspective to test how changes in drug prices alone altered the model results. Human immunodeficiency virus (HIV) was selected as the therapeutic area for the BIM, based on the high expense associated with treatment and evidence of unit drug costs increasing over time. The model estimated the total pharmacy costs associated with treating stable adults with HIV-1, and the impact of market share fluctuations over 1 year. Comparators were branded agents and selected based on current treatment guidelines. Two cost scenarios were constructed using AWPs from 1/1/2017 and 1/1/2018. Market share, population size, and other non-cost inputs between the two scenarios remained constant. RESULTS: AWPs for the top 50 branded drugs increased by a mean of 10.0% from 2017 to 2018. AWPs for the comparators included in the BIM increased by a mean of 11.5% over the same time period. The 2017 and 2018 drug cost scenarios resulted in a per-member-per-month cost of -$0.030 and $0.011, respectively, which equates to a 136.3% percent change. CONCLUSIONS: Pharmacy BIMs are sensitive to changes in drug price. This analysis exemplifies the importance of regularly updating cost inputs for models. Caution should also be exercised with using extended time horizons in BIMs when potential future changes in drug pricing are unaccounted for or are unknown.