Huh? The exercise price for options is established when employees are granted stock options, which almost always occurs at the beginning of employment. Employees can calculate the total cost of exercise based on the information contained in the Notice of Stock Option Grant. You can and should ask for this information before you join a company.
If you are granted 100,000 options with an exercise price of $0.20, you know that the total cost of exercise (assuming full vesting) will be $20,000. The company's valuation could increase fifty-fold and it wouldn't affect the cost of exercise.
Companies with skyrocketing valuations can be precarious for employees who join late, but here too employees can calculate everything up front and they should take into concern liquidity risk when evaluating what their options are really worth.