What is the the cheapist upgrade to make your car faster?

It all depends on how fast are you trying to go. I would recomend starting off with basic mantnence to increase the preformance of a vehicle such as a fresh air filter and good spark plugs and wires. when seeking a major increase in preformance on a budjet i would have to recomend nitrous oxide which when used properly is also relatively safe for your engine. Used nitrous kits could be found on Craigslist for as little as $300