Medellín, Colombia is for one of South America's trendiest, but cheapest, hot spots. Travel from $219 RT from August - January from Fort Lauderdale, Orlando, Dallas, or New York*
If Cartagena is more your style, you can book airfare and 7 nights hotel for $537 per person (double occupancy) from Fort Lauderdale, Dallas, New York, Los Angeles, or Washington, D.C.
*note - for some departure cities, the cheapest airline is Spirit Airlines. There will be additional costs for seat selection, printing of boarding pass, and carry-on and checked luggage.