This deal keeps on coming back – US Airways offers $358.90 flights (bookable via http://www.orbitz.com) to Lisbon in July and August 2014.
Flights to Europe are expensive at this time of year and this must be one of the cheapest ways left to go transatlantic during summer.
There are a number of dates available. We found it for July 25th and August 14th. Use ITA Matrix 30 day flexible search to find more dates.