According to the U.S. Bureau of Labor Statistics, prices for wireless telephone services were -33.12% higher in 2000 than in 2017.
Cell service experienced an average inflation rate of -2.34% per year between 2000 and 2017. In other words, cell service costing $50 in the year 2000 would cost $33.44 in 2017 for an equivalent purchase.
Average pricing changed by -11.78% in 2000, significantly below the yearly average change for cell service during the 2000-2017 time period. Compared to overall inflation for all items in 2000 (3.27%), price inflation for cell service was much lower.
Raw Consumer Price Index data from U.S. Bureau of Labor Statistics for Wireless telephone services:
How to calculate the inflation rate for cell service between 2000 and 2017
Start with the CPI inflation formula:
CPI in 2017 / CPI in 2000 * 2000 USD value = 2017 USD value
Then plug in historical CPI values from above. The CPI for Wireless telephone services was 75.985 in the year 2000 and 50.817 in 2017:
50.817 / 75.985 * $50 = $33.44
Therefore, according to U.S. Bureau of Labor Statistics, the "purchasing power" of $50 from 2000 is $33.44 in 2017 (in the category of Wireless telephone services).