Just keep grinding through. That's all you have to do. You can't reach the end of any level first try, you have to keep going until you have earned enough to buy the same formulaic mods each time to allow you to reach the end. Strange scaling of icons means a 1950s sedan is the same size as a fire truck (and the zombies tower over you). It's fun for the first few minutes, then you realise the whole game is the same. I think it was conceived as a way of making you watch adds.