How should donors calculate fair market value of premiums?

Summary:

The IRS defines fair market value as the amount a willing buyer would pay a willing seller.

If you can prove the cost of a premium, and the donors give you more,
isn't the rest deductible?

It is not your cost that governs this decision, but the value to the recipient. John Taylor's reply to the original question explains further:

If you receive something tangible you must reduce the amount of any
deduction you claim by the amount of its fair market value which the IRS
has defined as being that amount that a willing buyer would pay a
willing seller. It is the written requirement (per the IRS) that us
non-profits make "a good faith effort" in determining what the FMV is
and report same, contemporaneously, to our donors.