The June 24 episode of SBS's 'One Night of TV Entertainment' exposed G-Dragon's royalties from his song compositions, which was estimated to be around 790 million KRW (~$720,000 USD) annually. 

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This particular broadcast featured a special corner which explored many things Big Bang-related, including the reasons for the group's fandom--attributed to their addictive music and vocal ability (perhaps their good looks and humor, as well?). The segment also focused on the fact that Big Bang make their own music, which led to the talk of how much G-Dragon makes for writing the songs. 

Taking into account that songwriting is only one of many activities that fills GD's deep coffers, the revelation doesn't come as much of a surprise.