Background: With the continued advent of new therapeutics, burgeoning costs and financial toxicity are more frequently a complication in comprehensive cancer care. At Levine Cancer Institute (LCI), a multidisciplinary Financial Toxicity Tumor Board has driven $55-60 million in cost savings for patients annually, and presented patient cases can serve as a continual impetus for iterative quality improvement and cost-savings research. One such example highlights the augmented risk of financial toxicity resultant from increasing utilization of front-line chemoimmunotherapy (chemo-IO) in ES-SCLC. Both atezolizumab and durvalumab in concert with platinum-etoposide chemotherapy separately portend an overall survival benefit. Retrospective analyses have failed to demonstrate a PFS or OS advantage between the two regimens. Amidst comparable efficacy, we sought to ascertain institutional variance in cost profiles and financial toxicity. Methods: Billing and demographic data for patients with ES-SCLC who received chemo-IO with durvalumab or atezolizumab in an LCI hospital-based outpatient infusion center were queried. Treatment dates encompassed October 1, 2019 through April 30, 2022. Patient costs were compared and exploratory analyses were performed related to patient demographics, patient geography (with financial characteristics per Census data) and bad debt proportions. Results: 211 patients received chemo-IO with atezolizumab, and 108 patients received chemo-IO with durvalumab. Medicare (+/- supplementary insurance), Medicare Advantage, and commercial insurance constituted 85% of payors. Institutional covenants restrict publication of commercial and supplementary insurance data; for compliance and generalizability, reported is the Medicare-only population. Medicare allowables of $6791 and $8429 were utilized per dose of atezolizumab 1200 mg and durvalumab 1500 mg, respectively. Amidst 20% patient responsibility with Medicare, durvalumab patient cost was $327.60 higher per cycle. Incorporating observed mean number of cycles of 5.20 for atezolizumab and 5.31 for durvalumab, an average course of durvalumab was $1836.56 more expensive. Assessing the entire population, 24% of patients in both groups met criteria for bad debt; however, mean quantity of bad debt was 62% higher with durvalumab ($8988.77) than with atezolizumab ($5555.11). Bad debt was disproportionately congregated in lower income zip codes (data to be displayed pictorally). Conclusions: Chemo-IO with durvalumab was $1834.56 more expensive per treatment course in ES-SCLC, with 62% increase in quantity of bad debt relative to atezolizumab. Future directions may focus on institutional interventions to mitigate financial toxicity related to therapy choice and geographic allocation of resources, as well as the assessment of cost variance in other solid tumors.