During 2020 a free, 3-month long, certificate-granting, online workshop was offered to increase undergraduates' knowledge, skills, self-efficacy, and interest in observational seismology and scientific computing. Course registrants were comprised of 760 upper-level undergraduates from across science and math majors, representing 60 countries. Performance data revealed 58% of registrants initiated the workshop by completing at least one assignment, and 30% successfully completed the full workshop. These rates were higher than most comparable large-scale, open-access courses. This study explores factors associated with persistence by examining registration, performance, and pre/post survey data through the lens of the expectancy-value theory (EVT). Results indicate that EVT constructs show moderate to weak associations with initiation, including intrinsic (p<0.001), utility (p<0.001), and attainment value (p=0.24), and expectancy (e.g., completion intentions [p<0.001], prior computing experience [p<0.001], seismology research experience [p=0.01], seismology self-efficacy [p=0.84]). Two expectancy factors, academic major (p=0.050) and prior geophysics coursework (p<0.001), showed weak associations with completion. Demographic factors such as race, ethnicity, and gender were not associated with initiation or completion, but primary language showed an association with attainment and intrinsic values. Dissuading factors or costs were also captured through surveys. The few responses from non-completers limited the analysis. However, an examination of acute and accumulated costs suggests they may influence persistence, especially at completion. Based on these results, interventions are proposed to enhance persistence in future workshops. These include increasing motivational messaging, tailoring course structure to mitigate avoidable costs, and where possible, increasing communication to set time and effort expectations and showcase prior participants overcoming costs.