$496 - Flights to Madrid Spain from Boston
British Airways and KLM Airlines are both also offering discounted flights to Madrid from Boston, but their price is 50 to 100 dollars more expensive.
The cheapest flights fly on Mondays, Tuesdays, and Wednesdays. There are cheap flights on other days of the week as well, but flight price tends to be 50 to 100 dollars more on these days as well.
Once in Madrid, its very easy to get around Spain, with many people heading East to Barcelona, North to San Sebastian, or South to Malaga.
I’d suggest looking into train rides from Spain to Portugal as well.
Here’s a full list of popular rail destinations in Spain.
Portugal is a beautiful country that can be relatively cheap as well. Its a big destination for folks from the UK.