The value of your incentives can roughly be estimated from the time the speaker has to spend for the recording: 2 minutes equals 1 EUR is a good rule of thumb (not counting the time to get to the recording location; just the pure time spent there).
Incentives for telephone recordings are usually at a lower rate
because the speaker can participate from wherever he wants to:
3 minutes equals
As a minimum incentive we consider the sum of 5 EUR6.7.