I think this is an android or windows phone glitch. There have been quite a few posts on it lately. But not many post on the same thread or provide much info.
This a android glich but it only looks like it has doubled some how mine was up to over 500k per hour and i tried to buy a medal then said not enough money.
I have seen this using another's phone I know it dose it on the evo. It dbls every time you switch from city to city or buy something. It is false $ from what I have seen you can't spend it and it will reset to your correct amount of $ when you do other actions.
I have seen this before on the android and am not sure if it has been fixed but it is not that important.. it is just a display error and isn't your actual money output