Editing A Business Economics View on EL (section)

=== Harvesting XP ===
Assuming Excavator Cloak and harvesting level of 75:

  Average harvest time of 1 mullein: 2,2 sec (minimum time is 2 sec, 10% more to consider events)
  Walk time for reselling 1 mullein:  0,2 sec
  Reselling in Irsis flower shop: 1 gc
  
  Total cost: 1,4 gc
  Base XP: 45 XP

32 XP per "gc" in harvesting when doing mullein! The gc value comes from the time loss when harvesting mullein instead of grinding gold via blu lupines.
Try other harvests for different results, however in general, besides fighting, harvesting is the most cost-effective task.

An interesting aspect is that you can get a higher number if you harvest minor stuff, since the time loss is less. If you want to go for more harvesting XP, the efficiency drops as you consume more and more time for just a few XP.

Thus, from an economical point of view, one should not spend too much time with harvesting heavy stuff, if you can use the time better. This does not take into account that harvesting can be done while being (partly) afk. More afk time won't improve any other skill.