For me, the most fun got out of the game is the driving. It's nice to go and buy a new car while you've not got them all, but collecting the cars is not where the game is at, IMHO.
So I don't care if the money is too easy to get. And I agree with Speed_Freak. Yes, you can get 1m relatively quickly by doing the 180mph speed challenge over and over, but once I've got that million I definitely don't want to get another million by doing the same thing immediately. And if I was only going to get 1m each day, I'd rather do the Millionaires Challenge. Yes it takes you three times as long (or more) to get the money, and yes there's a chance of not getting the money, but at least it's fun.