I think first of all it depends on the jail. It's not like you're just sitting in a room, not living. You're experiencing stuff, and it's prison stuff, and that can be hard to shrug off. How valuable is $8 million if you're too broken to enjoy it?

Second, it depends on if you can keep anybody else who is in jail from knowing that you're sitting on $400 million. Otherwise that info will be beaten out of you long before your sentence ends. Maybe that's OK if it's at the bottom of the sea.