For sure, Whitey is right about booking dosmetic air fares while you are in Colombia.  Yes, it is about $125 US
Check:  
http://www.avianca.com.co  and you will be surprised to find the different between booking in Colombia and the flight website (for example: 
http://www.expedia.com) from your country.
When I was in Cali last October, I made last minute booking to go to Santa Marta - the next day!  I was surprised to find how cheap it was!   But it is best to book in advance just in case the cheapest fare has sold out.
I have been to Colombia 6 times, and have travelled by:
Virgin Atlantic (London to Miami); American Airline (Miami - Cali) - once
Iberia (London to Bogota via Madrid) - 3 times
Air Comet (London to Bogota via Madrid) - once
Connential (London to Bogota via Newark) - once
Avianca - dosmetic