Unlike most forms of racing, putting money into mods in autocrossing will not have a huge impact. Like said above most of it is simply how well you can control your car. Sticky tires are the biggest difference maker.
I had my MazdaSpeed Miata for 3 years and never changed anything but tires, granted it's a well suited car for autocross but most of the changes I wanted to do went away the first time I raced it. It was too well balanced from the factory that I thought changing just one or two things would screw it up. Kind of why I got rid of it, I would've had to overhaul the whole thing to make it a lot better, and then I would've ruined what street manners it did have.
Now my Corvette is getting a complete Z06 suspension put on it before I autocross it for the first time and I'll adjust from there. It isn't what most people or I would recommend but I'm fairly familiar with both of the factory setups and the Z06 is what I wanted. This Vette was $10k cheaper then any decent Z06 anyway. Besides it's not like I'm new at autocrossing, and my personal goals involve getting good at driving a fast car, not really what I was doing with the Miata.