For a straightforward equity purchase, cost basis is simply what you paid: invest $25,000 for shares and your basis is $25,000. If that stake is later acquired for $100,000, your taxable gain is the $75,000 difference. Where it gets subtle is with SAFEs and convertible notes — your cost basis is the cash you put in, but the holding period and share details don't fully settle until the instrument converts to actual stock, which matters for both long-term capital gains and QSBS eligibility.
Tracking cost basis accurately is one of the most underrated parts of angel investing, because it directly drives your tax bill years later when an exit finally happens. Keep records of the exact amount invested, the date, and the documents for every position — including follow-on checks into the same company, which each carry their own basis and holding period. When a portfolio spans dozens of companies and multiple instruments, reconstructing this from memory at exit time is painful, which is why it's worth recording as you go.