Depends on what you drive. I've known people who have won (and/or placed in the top 3) at the Solo Nationals in $500 stock Civics spending little more than the cost of a couple of new sets of tires, a set of the lightest available (legal) Honda factory alloys, a new set of suspension bushings, and a set of used performance shocks.

That sort of thing requires that you do your homework and find a car that can be exploited to win its class, knowing how to set the car up, being able to do all of your own work, AND being inhuman behind the wheel.

It also helps if you live in KS and don't have to include thousands of dollars in travel expenses in your budget. As much as Kansas sucks for... well, just about anything... it's a good place to be an autocrosser seeking a National championship. As Jesse can tell you, attending higher level events to hone your skills is a must, and being in KS, you have access to regional events on THE site that the Nationals are run on as well as being within about 6 hours of a wide variety of Divisional, Pro Solo and National Tour events. You've got a lot more driving to do to get that kind of exposure if you're a Floridian.

Of course, even in that case... $500 car. $500 in parts. $1200 in tires. Plus entry fees to at least one autocross every weekend from March through September (I've seen these guys do two regional autocrosses in two different cities in one day!)... a minumum of $800 in entry fees. And then there's gas and lodging for the events that you travelled to... and food.

Jesse's right. (man, that's gotta be at least twice that I've said that!) You can't be Nationally competitive for $5k if you live in FL.

Then again... if you're that good, there's a fair amount of sponsorship money and prizes you can win to offset your costs. Not too many people are REALLY that good, though.