This study estimates the production cost benefits that would likely result from the creation of the WEIS Market and from extending the full Southwest Power Pool (SPP) Regional Transmission Organization (RTO) market to include the WEIS footprint. To assess these benefits, the Brattle team created a unified nodal production cost model of the WECC and most of the Eastern Interconnection, connecting the two models across the seven DC ties. The integrated Western Electricity Coordinating Council (WECC) and Eastern Interconnection model was developed in the Power System Optimizer (PSO) production cost simulation software. PSO is a state-of-the-art production cost simulation tool that simulates least-cost security-constrained unit commitment and economic dispatch with a full nodal representation of the transmission system, similar to actual RTO and Independent System Operator (ISO) market operations. PSO can generate hourly prices at every bus and generation output for each unit, which allow us to estimate changes in generation output, fuel use, production cost, or other metrics on a unit, state, utility, or regional level.