The only way to make that much money in a time as short as 20 minutes is to sell something that costs 400k or more.
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Just South of the Zamorak Altar in the level 38 wilderness. Dig at the west-southwest corner of the building. ( You will have a Zamorak Mage Fight )
About 5-10 mins
every herb takes 70 mins to grow
K means One Thousand So, 400k means Four hundred thousand
He is easy to kill only level 20, you have to be a woman in Runescape to get to him though, my guesses your doing the recruitment Drive Quest.