Travel from Central America (Guatemala) to Colombia cheaply?

Planning the itinerary of my round the world trip, I will travel from Tulum/Cancun/Coba to Antiga, Guatemala. My next stop will be Cartagena in Colombia.

The flights from Guatemala City are way too expensive at $600+ so what is the best solution since we can't use the roads all the way down. Any other place to fly from that would be cheaper? Boat?

Let me know, thank you.

Flights out of Panama can be less than half that price.

Here's another alternative altogether that I've done a few times, hopping around the Darien Gap with no flights at all...

http://www.tripadvisor.com/ShowTopic-g294073-i1708-k5562030-Backpacking_your_way_from_Colombia_to_Panama-Colombia.html

Good luck and have fun.

Cheers,
Terry