You would either have to look back in your records or contact the plan administrator.
Please see tis answer from ToddL
The "plan cost at annuity start date" refers to your total after-tax contributions in the plan, if applicable.
If you did not make any after-tax contributions into these accounts, enter "0" (zero) in the plan cost entry.
If you did make after-tax contributions and have a 1099-R, it may be listed in box 9b. If the information is not listed there, you must find it in your records or contact your annuity administrator. It will be beneficial to find this information, as it may reduce your taxable income.
