This paper introduces the Output Value Index (OVI), a longitudinal measure of creator compensation per unit of digital output in the microstock photography market, 2018–2025. Using five independent contributor anchors deflated to real 2012 USD, we document a decline in the observed earnings range from $1.15–$3.10 per file per year in 2018 to $0.85–$2.55 in 2025 — while platform revenue per download rose 59% over the same period. A within-contributor difference-in-differences using the June 2020 Shutterstock payout restructuring as an institutional discontinuity partially isolates a platform extraction effect from supply-side dynamics. A structural finding emerges from the data collection process itself: Shutterstock's Terms of Service enforcement from December 2020 has rendered post-cut disaggregated earnings data empirically inaccessible, constituting a documented mechanism of informational asymmetry. All claims carry explicit epistemic status (FACT/INFERENCE). No causal claims are made beyond the illustrative DiD.