Delta and Virgin Atlantic have round-trip flights from Dallas (DFW) to Madrid, Dublin, Amsterdam, or London (MAD, DUB, AMS, LHR) for $367-$506 with a connection. Most flights are between August to March. 7-day minimum stay required.
London is the most expensive fare in this group: $487-$506. All other destinations are less than $449.
A Carry-on bag and also a personal item is allowed for free on Delta. You can avoid paying for seat selection by accepting the randomly assigned seat at the gate.