Over 20 states and local jurisdictions in the U.S. have imposed e-cigarette taxes. It is important to evaluate how adult vapers, including those who also smoke respond to e-cigarette taxation. The purpose of this study is to examine factors associated with adult vapers' cost comparison between e-cigarettes and cigarettes (i.e., which one is perceived to be more expensive) and expenditures (i.e., dollars spent) on these two products. We recruited a nationally representative sample of 801 adult e-cigarette users in the U.S., who participated in an online survey in April-May 2023. Nested-ordered logit and ordinary least squares regressions were used in analysis. On average, adult vapers spent $82.22 per month on e-cigarettes, and dual users spent $118.77 per month on cigarettes. More frequent e-cigarette use, lower state-level e-cigarette taxes, age and exclusive use of rechargeable tank systems with e-liquids were associated with considering e-cigarettes as less expensive than cigarettes. Exclusive use of tank systems was associated with lower e-cigarette spending. Adults who used e-cigarettes more frequently preferred to report weekly (versus monthly) spending on e-cigarettes; among dual users, everyday smokers were more likely to think of weekly (versus monthly) spending on cigarettes compared to someday smokers. Among US adult vapers, tobacco use frequency and e-cigarette device type are closely related to cost measures; e-cigarette taxes are associated with cost comparison between e-cigarettes and cigarettes, suggesting potential financial disincentive for vaping. Policymaker may consider imposing differential taxes by e-cigarette product types due to their different costs to consumers. This study provides supportive evidence that e-cigarette taxes may impact how vapers compare the costs of e-cigarettes relative to cigarettes. Given different expenditures and cost comparisons by e-cigarette device types, tax and policy design may need to take into consideration of e-cigarette device types. We also found that indoor tobacco use restrictions in shared housing may reduce vaping, and future research is needed. The associations between socio-demographics and cost comparison and expenditures reflect known patterns in price responsiveness and vaping prevalence. Cost comparison and expenditures are related measures, which may further reflect product affordability.