The CityVille Money Vault costs $10 outright, and it will give you 90 City Cash over a 30 day period. The 30 day time limit starts on the first harvest, with each harvest giving you 3 City Cash over that 30 day period. This means that you'll need to be active within CityVille and play each day, at least long enough to collect your Cash, or you'll apparently lose whatever you missed.
In addition to this, the Money Vault acts as a community building, giving it a function even after the 30 day time period has elapsed. It adds 500 citizens to your city's maximum possible population, but adds no Downtown Value if you choose to place it there.
This Money Vault will likely only be available to purchase for a limited time, so if you're interested in doubling your money this month, make sure to buy one while you still can.
What do you think of the Money Vault in CityVille? Will you invest the $10 purchase price to receive 90 City Cash after 30 days? Let us know in the comments!