Cost basis is the total original cost including tax-related adjustments.
Original cost: Usually the purchase price counting any associated purchase costs such as a trade commission.
Tax related adjustments: For example, your cost basis may be adjusted for a corporate action (such as a stock split), wash sales, dividends, or capital gains distributions.
Cost basis must be determined per share in order to calculate the taxable gain or loss on the removal or sale of any security reported to you on the 1099-B.
A qualified tax advisor can also assist you in calculating the actual cost basis of an asset and in any subsequent gain/loss calculations.