You would need around $840.00 (€720.07) 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 $24.84 less in Portugal ($50.16) than in United States ($75.00).
For monthly supply of groceries worth $250.00 in United States you will pay $105.00 less in Portugal ($145.00). If you are an average smoker (1 pack per day), you'll spend $30.52 less on cigarettes each month than in United States.
While using public transportation in Portugal using the monthly pass you'll end up paying $28.45 less than in United States. However, if you will be driving a car 20 mi on average each work-day, each month you'll spend $53.41 more on gasoline than in United States.
To buy an 915ft2 apartment in Portugal city centre you will need $164,067.65 which is $26,821.10 less than in United States ($190,888.75), however to buy one outside of the city centre you will need $110,691.60 which is $21,666.15 less than in United States ($132,357.75).