When you exercise an employee stock option, you need to pay the strike price of the option to buy the stock. The exact cost to exercise equals the number of options you exercise multiplied by the strike price of the stock option.
You may be able to cover the cost several ways, including with cash, with other shares (known as a stock swap), or via methods like a share withholding, cashless exercise, or a sell to cover.
Ultimately the decision you make will be tied to personal financial planning goals and company plan rules and regulations.