what is "plan cost at annuity starting date"?

[JamesG1]

Are you reporting an IRS form 1099-R and using the simplified method to compute the taxable amount of the distribution reported in box 1?

If this is the case, you are being asked information about your contribution to your pension plan.  Your 'plan cost at annuity starting date' is your contribution to the pension plan.  Your contribution will be recovered each year as the nontaxable portion of your pension distribution.  

In some cases, your 'plan cost at annuity starting date' is reported in box 9b of the IRS form 1099-R.  Or you may have to contact the issuer of your pension distribution to determine this amount.