Yeah as Pi said GTAV lifetime sales are announced every 3 months and it is correct.Yeah most of the time it goes on sale its only down to €30 lol. Sure I remember the ps2 version of San Andreas being cut to 20 during the ps3 days and still selling well.
$6bn is probably a low ball honestly. GTAV has been full price most of its life, and GTA Online rakes it in.
It will probably be ps5 days before you can get gta v digitally for 20 on ps4.