You would need around $885.01 (€719.69) in Portugal to maintain the same standard of life that you can have with $1,500.00 in United States (assuming you rent a place to live in both cities). This assumes net earnings (after income tax).
If you eat out 4x a week in an inexpensive, and 1x a week in a mid-range restaurant you will pay $22.13 less in Portugal ($52.87) than in United States ($75.00).
For monthly supply of groceries worth $250.00 in United States you will pay $97.50 less in Portugal ($152.50). If you are an average smoker (1 pack per day), you'll spend $21.62 less on cigarettes each month than in United States.
To buy an 915ft2 apartment in Portugal city centre you will need $172,949.47 which is $17,939.28 less than in United States ($190,888.75), however to buy one outside of the city centre you will need $116,683.90 which is $15,673.85 less than in United States ($132,357.75).