N.Y. Comp. Codes R. & Regs. tit. 20, § 3-3.5 – Fair market value.

(a) The fair market value of any asset owned by the taxpayer is the price at which a willing seller, not compelled to sell, will sell and a willing purchaser, not compelled to buy, will buy.
(b) The fair market value, on any date, of stocks, bonds and other securities regularly traded on an exchange, or in an over-the-counter market, is the mean between the highest and lowest selling prices on that date. If there were no sales on the valuation date, such value is the mean between the highest and the lowest selling prices on the nearest date, within a reasonable time, on which there were sales. If actual sales within a reasonable time are not available, the fair market value is the mean between the bona fide bid and asked prices on the valuation date or the nearest date within a reasonable time.
(c) If the actual sales prices or bona fide bid and asked prices within a reasonable time are not available or if by reason of the character or extent of the taxpayer's investments or for any other reason such prices are not truly indicative of value, the fair market value is ascertained as follows:
1. (1) in the case of shares of stock, on the basis of the issuing corporation's net worth, earning power, book value, dividends paid, and all other relevant factors;
2. (2) in the case of bonds and other securities, by giving consideration to various factors, including the soundness of the security, the interest yield, and the date of maturity.
(d) If a taxpayer consistently computes the fair market value of its stocks, bonds and other securities on some other basis, such as the last selling price on the valuation date, such method of valuation may be accepted by the Commissioner. In all such cases, a complete explanation of the method of valuation must be included with the report.