The cost of the outward trip from Porto Airport to the city is usually between 20 to 30 Euros. Most taxis here are metered and therefore haggling over the cost is really not an available option. You should note that the rates go up by 20% in the evening hours, as from 2100hrs to 0600hrs, and these changes are automatically reflected by the taxi meter. The surcharges also apply to weekends i.e. Saturday and Sunday and the public holidays. There are some companies from which you can negotiate a good rate for a familiarization trip around the city though.
You will be pleased to know that most taxi drivers here have a good command of the English language plus they are friendly and helpful. With several renowned taxi companies such as Greenbus, Algaserra, Suntransfers.com, Holiday Taxis and Transcor, among others, you certainly have a wide variety to choose from. Should you be planning to visit the wineries it is necessary to take a cab to-and-from these locations because the bus routes do not cover those areas.