I need to cash out my 401k for to pay for emergency expeneces. What will this cost me

Too much!

Your 401K is the very, very last thing you touch. First, not all plans permit inservice withdrawals. If your plan does permit it, there will be a penalty of 10%. In addition, any funds you withdraw will be added to your taxable income for the year. This means anywhere from 20-40% will be lost to taxation. Plus there is the loss of investment income on the funds.

Check if you plan has a loan provision. This is usually a much better alternative.

Check with your plan administrator.
Many 401(k) plans allow employees to make a hardship withdrawal because of immediate and heavy financial needs. Generally, hardship distributions from a 401(k) plan are limited to the amount of the employee's elective deferrals only, and do not include any income earned on the deferred amounts.