The usual way to get in is from Manizales. From there you have several options: Go on a tour and jump off (120'000 COP incl. entrance and breakfast), Taxi (300'000 COP) take a bus to the intersection and hike/hitchhike up. The last option is the best one if you have the time, as you would gradually acclimatise to the altitude, not pay excessively, and avoid the extremely commercial feel of the Concesion.
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