CASTLE BIOSCIENCES INC (CSTL) stock intrinsic value fair value estimate & owner earnings analysis

Castle Biosciences sells genomic tests that help doctors decide how to treat skin cancers (DecisionDx-Melanoma, DecisionDx-SCC), Barrett's esophagus (TissueCypher) and other conditions. Revenue grew from $52M in 2019 to $344M in 2025 (2025 +3.7%; first half of 2026 +7.5%), but the growth has been paid for with stock. Stock compensation was $46M in 2025, and we treat it as a real cost. Owner earnings (operating cash flow, less working-capital swings, stock compensation and ongoing maintenance capital spending) were deeply negative from 2020 to 2023, about $0.54 per share in 2024 and only about $0.06 per share in 2025, an ~89% drop. Why it fell: much of 2024's cash came from Medicare collections on DecisionDx-SCC, and Medicare coverage of that test was discontinued in April 2025, which the company says will keep reducing revenue and cash unless its reconsideration requests succeed. First-half 2026 figures (operating cash flow before working capital of roughly $12M against $21M of stock compensation) point to a further step down, so we do not treat 2024 as the normal level. Accounting judgements: we excluded the company's own new headquarters building ($9.3M in 2024, $31.0M in 2025, $42.8M total, finishing in early 2026) from maintenance spending, since it is a one-off discretionary project, and we did not add back stock compensation. Depreciation and amortization jumped from $16M to $41M in 2025; this is non-cash and already reflected in cash flow, but we did not assume it implies growth. Base case: owner earnings hold at roughly $0.10 per share (between 2025 and the 2024 peak, tilted to the lower recent figures) for ten years, then fade to zero over the following decade because we cannot name what sustains earnings beyond that. We credit no growth: revenue has grown, but turning that into owner earnings would require margin expansion or new products, which are not yet proven. Year-one growth assumed is zero versus a trailing decline of about 89%, so no rebound is assumed. Value about $0.90 per share. Bear case: Medicare coverage of SCC is not restored and cost growth keeps pace with revenue, so owner earnings stay at or below zero. Cash and only $10.2M of debt mean the company is not worth less than nothing, but nothing in the earnings supports value: $0 per share. Bull case: the business returns to the 2024 level of about $0.55 per share (already achieved once, with stock compensation now lower than in 2023), held for ten years then fading: about $5 per share. What could prove this wrong: the company holds a sizeable cash and securities pile that we do not add to value on top of owner earnings, and a revival of Medicare coverage or steady revenue growth against flat stock compensation would move results toward the bull case. Conversely, a further slide in first-half-2026 cash generation would push toward the bear case.