the approach depends a bit on the angle through which the question is asked.
If it is about "how should a taxi license be priced", then a very logical approach would be to estimate the willingness to pay (WTP) of a taxi driver. In short, this WTP is equal to the value which a person can generate with taxi driving of a focal time frame (here I would assume several years, but check with interviewer) minus the value of his best alternative income source over the same time frame (to be inquired with interviewer (might also be zero)).
So in essence, you need to estimate the profits for the taxi driver and subtract the best alternative income. This will give you the maximum price that can be sustained in this market.