How do I find the answer to the "Plan cost at annuity starting date"?

The “plan cost at annuity starting date for the simplified method” is a number that is used to calculate the taxable amount of the distribution for the current year. 

The “plan cost” refers to the after-tax contributions that were made to the retirement plan.  If the employee did not make any after-tax contributions to the plan, then the plan cost will be $0 and the entire distribution will be taxable income.  If there was any after-tax contributions made to the plan, then a portion of every distribution will be non-taxable. 

If the current tax year is the first year that a distribution is received from the retirement plan, then the plan cost is likely to be reported in box 9b of Form 1099-R, 1099-CSA, or 1099-CSF. 

The start date for the annuity is simply the date that the first distribution check was received. 

Once you have determined the plan cost and the starting date for the annuity, keep the information for your records.  If you do not have the information available, your plan administrator may be able to provide it for you.