It’ll stay in your account permanently if you get in there before they all disappear. After all five million are handed out, it’ll be back to the regular price.
This giveaway comes a few weeks after the news that Payday 2 will be getting VR support. So far, that’s just announced for the Vive. But if you own one of those and want to pursue some heists in VR, then this giveaway may be right up your alley.