From Rocky Mountain Institute's Executive Summary: "In this report...we examine the cost-savings opportunities renewables price declines have made possible for Tri-State Generation & Transmission Association and its member co-ops. Specifically, we consider their opportunity to engage in large-scale procurement of cost effective renewable energy projects, while maintaining system reliability requirements. We analyze two illustrative power supply portfolios based on publicly available data, and find that procurement of new wind and solar projects represents approximately $600 million of cost-savings potential for Tri-State’s members through 2030, versus continued reliance on legacy coal-fired generation. Scaled adoption of renewable energy by Tri-State could also mitigate risks of revenue loss and cost increases associated with reliance on existing assets for electricity supply, reducing the rate increases under a range of risk scenarios by 30% to 60%. The analysis presented in this case study illustrates that immediate collective action between wholesale energy providers and member co-ops can mitigate risks, identify regionally appropriate solutions, and leverage aggregate buying power, enabling an efficient and equitable transition toward a more cost-effective energy supply mix."
You can find the report here and in the left side bar under "Download". There is a related resource below "Keeping Rates Down Through Cost-Effective Energy" that is a 2-page handout that summarizes the report.