How can it be answered?
Perhaps the most logical thing is the dear, old and very appropriate "DEPENDE".
It depends on what I'm looking for and what kind of life I want to lead, on the place where I settle, on the things I do not want to give up and on the standard of living that I want to keep up.
Really, our reply would be the same if someone asked us how much it costs to live in Italy. There is a difference between the cost of living for those living in the centre of Bolzano and those living in a tiny town in the Molise area. Even in our country there are families that need a monthly income doubled or tripled in comparison to what others survive on.
Let's start, for example, with the management of a car. Transfer of ownership, stamp and insurance have extremely low prices compared to Italy. Also, if you learn to buy wisely, you can save money on food too. The municipal markets offer quality meat and fish at very reasonable prices. Even the supermarkets have butchers and fishmongers inviting to the eyes and the wallet. Portuguese wine (alentejano in particular) has a very good price / quality ratio. Usually the cost of a coffee in a regular café is between 60 and 85 cents (and sometimes I have drank worse in Italy).
If you know where to go, you can eat fresh fish at a restaurant, accompanied with a good glass of wine for less than 20 euros.
Of course this means not stopping at the first place for tourists but getting a little more familiar with the local culture, interacting with locals and maybe having a menu only in Portuguese. It also means exploring and having an open mind and maybe being willing to make some concessions. I can also find Parmigiano 24 months, mozzarella bell and chianti classico, but obviously I pay them much more here than in Italy.
So how much does it cost to live in Portugal?
Come for a few weeks, try to get to know and get absorbed into the local culture. You will give yourself the answer and it will be the best you can ever find.