Air France and KLM have round-trip flights from Chicago (ORD) to Porto (OPO) for $512-$521 with a connection. Flights October to March. Extended weekends available. Flight may operate as a Delta code-share.
Fare allows for a free carry-on bag and personal item. You can avoid seat fees by accepting the randomly assigned seat at check-in.
Avoid the inferior TAP Air Portugal fares at this price. TAP fares are fare more expensive after you add in baggage and seat fees.