Parque da Estação de São Bento: on the right side of the train station São Bento, in the city center, 10 euros a day.
https://www.empark.com/uk/pt/aparcamento/porto/parque-estacao-ferroviaria-de-sao-bento/
Parque da Alfândega: at the River edge, near the old customs'house, 20 euros for 72 hours (in/out when you want) (no link, sorry)
These are the best, for what I know.
Up hill at Clérigos area, there is "Irmãos Rocha", between 12 and 16 euros a day, Rua Miguel Bombarda.
Or you can just leave car at the airport and take the metro to the city. There, parks are around 4/5 euros per day.
There are places inside the city where parking isfree, even at the city center, but car with a foreign plate or with something saying they are from a rent-a-car, are, for sure, robbed. You would be facing, at least, a broken glass.