The $50,000 price is likely a tax lien or second lien.....you still need to worry about the $500,000 first lien.....If it looks too good to be true, it normally is.
There are foreclosures around that sell at a discount, just not that big of a discount. Usually they're somewhat ugly and need some elbow grease to get them looking good again. If you want to buy a fixer they may be good deals. If you want to buy one that looks great and ready to move in, they'll be closer to market price.