This achievement is really just a gimme ... i got this in the first few hours. 500,000 is only twice the amount you can carry at max without a upgraded wallet. You get money from looting dead bodies, selling loot, opening chests and finishing quests like helping out allies under attack or people stranded with a broken down vehicle.
Some of the guns you buy are upwards of 200,000 plus ect and then some, within refilling ammo and buying armour/health items you'll have this in no time.
Simple as that.