You would need around $855.00 (€721.03) 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 $24.02 less in Portugal ($50.98) than in United States ($75.00).
For monthly supply of groceries worth $250.00 in United States you will pay $102.50 less in Portugal ($147.50). If you are an average smoker (1 pack per day), you'll spend $27.81 less on cigarettes each month than in United States.
While using public transportation in Portugal using the monthly pass you'll end up paying $27.77 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 $54.79 more on gasoline than in United States.
To buy an 915ft2 apartment in Portugal city centre you will need $166,774.83 which is $24,113.92 less than in United States ($190,888.75), however to buy one outside of the city centre you will need $112,518.05 which is $19,839.70 less than in United States ($132,357.75).