Put a range on it

False precision is not required. A defensible range is.

Use available records, direct observation, and the estimates of the people doing the work. Separate what you know from what you assume. If the annual cost might be between $40,000 and $120,000, write the range and its assumptions. Do not turn the midpoint into a fact.

The range has two purposes. It prevents a small inconvenience from becoming a strategic program, and it places an upper boundary on what a solution could rationally cost.