Cartagena-Colombia-Travel

Hello,

I have been checking out flights from SCL to Bogota and then on to Cartagena. We are planning a short family reunion in Cartagena and then some further travel within Columbia ( Cali, Medellin, Bogota, Villa de Leyva, Barichara, for example ) before returning to SCL about a month later. So far my research has suggested that if I wanted to fly to Cartagena on the way in, and then return from Bogota, the two one way flights would be double the cost ($1000)of a return flight to either destination($500) . Makes no sense to me! Any ideas?Second question: If you had to choose an airline between Avianca, Copa and Lan, which one would you choose?

Please help

I didn't find the right solution from the internet.

References: 

https://www.allchile.net/viewtopic.php?t=14239

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Thanks