The insane addition in the USA is due to so many middle men working to up the cost basis to increase their bottom line, essentially the insurance companies are running a racket.

Here is the associated research report that the link I provided used for their data aggregation and comparisons:

https://www.rand.org/content/dam/rand/pubs/research_reports/RRA700/RRA788-1/RAND_RRA788-1.pdf

These are the associated surveys:

https://www.t1international.com/access-survey16/

https://www.t1international.com/access-survey18/

https://www.t1international.com/access-survey/

Specifically for your question about cost basis it seems they tallied the total price for the insulin based on its chemical costs (for each type of insulin) and then cross referenced that with the price placed down on both prescription drugs and over the counter drugs listed in the IQVIA MIDAS database:

"In our analysis, we used prescription-drug market data from IQVIA’s MIDAS database.2 The data we examined cover the year 2018 and span 33 Organisation for Economic Co-operation and Development (OECD) countries, including the United States.3 We did not restrict our analysis to prescription drugs, as certain insulins are available over the counter in some countries (including the United States)." - RAND RRA788 PDF Page 2, Paragraph 2; Mulcahy, Schawm & Edenfield "Comparing insulin prices in the United States to Other Countries" 2020.

(IQVIA MIDAS: https://www.iqvia.com/solutions/commercialization/brand-strategy-and-management/market-measurement/midas)

Once they had a general understanding and baseline for what the products should cost they then indexed and compared prices globally using volume weighted pricing, they do note specifically that the manufacturer prices they used from MIDAS does not reflect rebates or discounts that would exist at point of sale (RAND RRA788 Pg 3 Para 2).

These are the references the research paper provided:

Cefalu, William T., Daniel E. Dawes, Gina Gavlak, Dana Goldman, William H. Herman, Karen Van Nuys, Alvin C. Powers, Simeon I. Taylor, and Alan L. Yatvin, on behalf of the Insulin Access and Affordability Working Group, “Insulin Access and Affordability Working Group: Conclusions and Recommendations,” Diabetes Care, Vol. 41, No. 8, August 2018, pp. 1299–1311.

Fuglesten Biniek, Jean, and William Johnson, “Spending on Individuals with Type 1 Diabetes and the Role of Rapidly Increasing Insulin Prices,” Health Care Cost Institute, January 21, 2019. As of September 1, 2020: https://healthcostinstitute.org/diabetes-and-insulin/spending-on-individuals-with-type-1- diabetes-and-the-role-of-rapidly-increasing-insulin-prices

IQVIA, “MIDAS,” webpage, undated. As of September 1, 2020: https://www.iqvia.com/solutions/commercialization/brand-strategy-and-management/market- measurement/midas

IQVIA Institute for Human Data Science, Medicine Use and Spending in the U.S.: A Review of 2018 and Outlook to 2023, Durham, N.C., May 9, 2019.

Roehrig, Charles, The Impact of Prescription Drug Rebates on Health Plans and Consumers, Ann Arbor, Mich.: Altarum Institute, April 2018.

Tribble, Sarah Jane, “You Can Buy Insulin Without a Prescription, but Should You?” Kaiser Health News, December 14, 2015. As of August 20, 2020: https://khn.org/news/you-can-buy-insulin-without-a-prescription-but-should-you/