If you took Rick's Spain trip, it starts in Barcelona but ends in Seville. The flights out of Seville are limited so did you take a train back to Barcelona to keep a true round-trip flight?
Also, if you took this trip, was dinner early on the last night? Thinking about leaving that evening instead of after breakfast when the tour officially ends.
I'd check the prices out of Madrid instead going all the way back to Barcelona if you can't find a reasonable fare flying back from Seville.
The last dinner is unlikely to be early, it is Spain after all, and I wouldn't want to forfeit a paid for hotel room.
You might want to look into multi-city (sometimes called open-jaw) flights. They are NOT 2 one way flights, and are often cost efficient.
I took the train from Seville to Madrid, stayed one night, and flew home from Madrid, which at the time offered me the most flight options back to west coast USA. Seville does have an airport so you can try looking at options on sites such as Skyscanner. Depending on your home airport, there may be good options. Just be sure to look at details closely and give yourself a lot of transfer time if there are separate tickets involved (that is, the tickets are not all through the same airline or its partners).
The dinner on the last tour night is usually a very nice celebration to wrap up the tour. It is "early" for Spain dinner time (6-7ish) but not for an American early dinner (4-5-ish). When I did this tour I recall it was a pretty late night including a carriage ride after dinner. I personally would skip a breakfast rather than a nice dinner and hotel stay but of course choose what works best for you. Just make sure the guide knows so they don't book dinner seats for you and can release your hotel room a day early (the hotel would hold your bags for you).
We had the same issue looking for flights back to the West Coast, US from Seville for our upcoming October trip. We ended up adding two nights to the tail end of the trip in Madrid. We're taking a train from Seville to Madrid, spending a final night or two in Madrid, then getting an early flight out of Madrid. We're bouncing through Heathrow on our way back West, but there were a LOT more options from Madrid, flightwise.
Reading the description of the final evening, I would not skip it.
Reiterating the advice to look at 'multi city' ( aka open jaw) flights. Home-Barcelona, Seville-Home.
Certainly no reason to go back to Barcelona for a flight home.
A cursory search shows the last train from Seville to Madrid leaves before 9 PM, so attending the final dinner and leaving that night does not look possible.
