Please take the information on this page with a tiny grain of salt. I think it's all accurate, but I might have got something wrong. Let's hope not O:-)
This is what taxis look like, in Portugal:
You can also find taxis with the former colours: black taxis with green rooftops. They work the same way as the other ones (three pedals and so on).
Regarding the sign on top of the taxi (which can be seen on the next photo), it contains:
If you're taking the taxi inside the city of Braga, a taxi should cost something between 5 and 10 euros, depending on where you're going (and what time it is, perhaps). Going to Bom Jesus might cost a bit more, as I believe they charge a different toll when they get to the mountain.
If you're taking a taxi from the Oporto Airport to Braga, for instance, you can ask the driver how much he will charge you before you get in the taxi. The service is charged in kilometers, he checks his table to see how much it is (unless he knows it by heart) and tells you. AFAIK, this service should cost around 50 euros.
If you have luggage for the trunk, the ride costs 1,60 euros more.
If you called the taxi by phone, the ride costs 0,80 euros more.
If it's during the night or weekend, the driver can charge 20% more (this is really not usual, but it is legal).