That's because in missions there is an 'unlimited' number of vehicles. In Freeplay, however, you must buy new vehicles to gain access to them, and there is a certain number of parking spots. Each time you buy a vehicle, the price goes up due to interest, so one unit could potentially bankrupt you. I think it's gay that you have interest myself. Like one vehicle that says it costs 400,000 Credits in the order screen drains my 50,000,000 Credits. Very unrealistic.