Hi,
If you won't go through the US, the main routes will be through MAD (Iberia, Air Europa), or CDG/AMS (AF/KL). From certain cities, you might also find routes with Avianca or Tam, but it's not certain it will be cheaper.
Generally speaking, fares under 600 euro happen, but they are not very common - and almost impossible if you go in December, as christmas-new year is one of the busiest times.