In general the basic capital gain rate applies (see the previous question). However, if the stock is qualified small business stock (also referred to as Section 1202 stock) and you acquired the shares directly from the corporation for cash, property, and/or services after September 27, 2010, there’s zero capital gains tax; all of the gain is excludable from gross income. Only certain types of C corporations can issue Section 1202 stock, so check with your tax advisor.
Generally, the amount paid for property. You need to know your basis to figure gain or loss on a sale.