taxi is very cheap, maybe the cheapest in europe, except when you catch it at the airport. be careful because there are some taxi drivers that are not as nice as you would like.
if you're going from airport to your hotel ask the hotel before you leave your home how much it is for the taxi run.
portuguese people anyway are extremely nice and polite and always ready for help and directions

https://www.smartertravel.com/2017/02/13/taxi-rip-offs-lisbon-warnings-dangers/

http://turismo.dnoticias.pt/artigo/5854-46-taxistas-aeroporto-lisboa-apanhados-enganar-turistas

edit: give a look here

https://lisbon-portugal-guide.com/lisbon-transport/lisbon-taxi-guide.html