According to the U.S. Bureau of Labor Statistics, prices for milk were 31.23% lower in 2000 than in 2017.
Milk experienced an average inflation rate of 1.61% per year between 2000 and 2017. In other words, milk costing $5 in the year 2000 would cost $6.56 in 2017 for an equivalent purchase.
Average pricing changed by 0.15% in 2000, below the yearly average change for milk during the 2000-2017 time period. Compared to overall inflation for all items in 2000 (3.27%), price inflation for milk was much lower.
Raw Consumer Price Index data from U.S. Bureau of Labor Statistics for Milk:
How to calculate the inflation rate for milk 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 Milk was 107.742 in the year 2000 and 141.385 in 2017:
141.385 / 107.742 * $5 = $6.56
Therefore, according to U.S. Bureau of Labor Statistics, the "purchasing power" of $5 from 2000 is $6.56 in 2017 (in the category of Milk).