Upgrading your cart in this game is pretty simple. There are only 3 upgrades in 4 of the categories, and then just 1 or two in 3 more categories. The usually cost about $350 to $400 each upgrade. The weapons are cheaper and the paint upgrade is under $100. Each race can gain $100 - $200 and I flew through the upgrades are each course set. I didn't find what exactly paid more for what you were doing. Some experience is gained but I didn't see a direct correlation between the experience gain and money. I could not do this in just the two play throughs. I played one character and replayed just a few missions to maintain gold. I ran through it with the reaper, and then had to go through 2 more course sets to get my last two upgrades. I didn't see any way to gain coins other than playing the story missions.