United has DC, Houston and Denver to Barcelona available from $437 roundtrip this fall.

Most booking sites will show you fares starting at $545, for instance here’s an availability calendar from ITA Software:

However airfare metasearch site Momondo will help you find even better pricing than that.

The fare is available for departures between October 1 and December 12. There’s a $300 change fee and a Saturday night stay is required. You’re not going to upgrade these tickets.

In addition to search these fares using Momondo I have two pieces of advice to offer.

First is credit these United flights to Singapore Airlines KrisFlyer. While a non-elite MileagePlus member crediting United flights to the revenue-based MileagePlus program will only earn only about 1500 miles (5 miles per dollar on the cost of the ticket excluding taxes), nearly all United fares earn 100% of flown miles when crediting to Singapore Airlines KrisFlyer (.pdf). That means you earn around 8000 to 10,000 or more Singapore Airlines KrisFlyer miles for the trip flying United (depending on the route you take) and crediting this K fare to KrisFlyer.

While Singapore miles do expire after 3 years (not 3 years of inactivity), you can top off a KrisFlyer miles account with transfers from American Express, Chase, Citi, and Starwood.

Second is watch your wallet on Las Ramblas.

(notice the young boys peering in)

(HT: Airfare Spot)