Without taking several days on a train - fly.
Depending on time of year and flexibility - you can find lots of cheap flights. Both IB and Alitalia fly this route. IB sucks on long-hauls but isn't too bad for these shorter ones.
Try the usual US internet sites. You can also try
http://www.despegar.com and
http://www.viajeselcorteingles.com Both these sites are in Spanish but I'm pretty sure despegar.com has a page in English that you can select.
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