Enough to pay for it 2 times over. In other words; you should have enough money to not only purchase the mansion, regardless of the size or location, but to also pay property taxes, utilities, upkeep, and repairs.
It depends on your location, you have not mentioned where do you live and which type of home you need. It also depends on how money you own. If you are having a good amount of money you can get a good mansion, if not then you have to save some money to have it. You can take the advice of some financial advisory firm they can help you better.