Now that I am Boston-based, I love seeing cheap airfares from my home city! Per Loyalty Traveler and some research on my own, you can get flights as low as $343 from Boston to both Madrid and Barcelona. You can fly to other cities within Spain for slightly more as well. Cheapest prices are available in the Winter and Spring, but you might be able to still find a relatively inexpensive flight over the summer.
These fares are with United, Air Canada, and American Airlines. You can always use Kayak to search for all airlines and a range of dates. Some flights are direct, while others have layovers. Madrid is slightly easier to get to then Barcelona, but both are still do-able and inexpensive!
Barcelona is one of my favorite European cities, so if you have’t been I highly suggest it! Anyone book a cheap flight? If so, enjoy!