No stop in Madrid!! Shame on you!! ;)
You are leaving things a bit late. However, for an arbitrary date of March 11th, you can still find one-way tickets on Spanish discount airline Vueling for about 35-65 Euro (all in), depending on which of the seven daily flights you choose.
Tickets on the AVE can be cheap if you purchase them in advance. For that same arbitrary March 11th date, I found second-class "Web" specials for 75-80 Euro each, one way. The Spanish national rail website, RENFE can be a bit of a chore if you don't know Spanish, but several of us can help, or you can use translate.google.com and you should be OK. You'll have to purchase your tickets in two parts....Barcelona-Madrid and Madrid-Sevilla...coordinate your arrival at Madrid's Atocha station with the departure (from the same station) for Sevilla. You can print the tickets at home.
Whatever option you choose book it ASAP.