Hi there, I just found the solution for the Prius disappearing in Monopoly for iPad =)
I think the car began to disappear with the April 2012 update, but there's a way to avoid it disappearing
Steps:
1. Install i-FunBox
That's a free tool for PC/Mac to view iPad/iPhone/iPod files, backup&restore them.. it can be useful to do full backups of Apps, in-App purchases, configuration files, completed games, installing/removing apps and more
2. Open the directory MONOPOLY/Documents
If you already played the game and the car has disappeared, there should be in that directory the file "monopoly_info.plist". Inside that textfile there is a line which says something like token-prius-expiration 2012/04/28 but.. if you edit it, it will be restored again to that date.. so..
3. Delete the file 'monopoly_info.plist' and create a folder with that name
You know it's not possible the existance of a folder and a file with the same name so it won't be created again. There is no crash after doing this and the car comes back!
That's it..
Enjoy the game
=)