For each listing with a usable year, we estimate age in years, add one to avoid divide-by-zero on nearly-new cars, then spread price over months. The KPI shows the mean of those monthly rates across the slice.
MSRP-aware alternative
When manufacturer suggested retail price (MSRP) is available for a model, a more intuitive formula is (MSRP − fair market price) ÷ age in months. We prefer that where data exists; the sidebar may still show the listing-derived mean when MSRP is missing.