Hola Kuba,
Try the Sinbad Coach lines:
Private Travel Office SINDBAD was established in April 1983, and now is one of the oldest and the biggest travel office on Poland. Our main activity is an International Coach Line transport, outgoing and incoming tourism, and sport events.
Nowadays SINDBAD is one of the biggest coach transport company in this part of Europe, and for certain a leader in this business in Poland. We have over 70 comfortable, well equipped coaches SETRA, with the averate age 2,5 years, we employ over 300 people, and as a tour operator we offer a holidays in Spain, Croatia and the Island of Thassos. Looks like a one way ticket is from 149 euro from Madrid.
http://mail.sindbad.pl/rozklady/r7.pdfIberia - Santiago to Warsaw 360,78 euro
Spanair 760 euro
Looks like the Santiago to London - London to Warsaw is your best bet.
Good luck Kuba - hope everything is OK in Poland?
Sil