Cheap is possible with each of these mods.
There are some people who can buy a car and be content with it in its stock form. Others are happy for a while but then get bitten by the speed bug and feel the need to start tinkering. Sometimes this bug bite leads to a thin wallet, or, in the worst case scenario, a car wrapped around a tree in the woods. While we can't help with the whole car around a tree thing we have found some simple and relatively inexpensive mods that'll let you boost your car's performance without breaking the bank.
Okay, a $200 cold air intake may not exactly meet the "inexpensive" requirement we've set. But it is simple enough to hook up, taking about as much time under the hood as a commercial break on TV. A cold air intake is not able to free up a ton of horsepower. But, it will increase airflow to the engine and its position outside of the engine bay will allow it to suck in denser cold air. In other words the engine will breathe easier and therefore perform better.
You want to upgrade your suspension but it's going to cost thousands of dollars for a set of decent coilovers. A car owner looking for upgraded suspension is sometimes just going for a stiffer setup, and that can be accomplished with an investment of only a couple hundred bucks. The answer is polyurethane bushings. Out of the factory most cars have rubber bushings, which over time stretch and become soft, providing a handling setup that can feel a little loose. Polyurethane bushings aren't too expensive and will stiffen the suspension much more so than stock rubber bushings. When buying a used car take a look at the bushings. If they're rubber and terrible, replace them.
Installing new spark plugs is a bit of an underwhelming suggestion, we know. But they are cheap and while they won't increase performance they will help you recapture lost power. Every engine is different, and each one will typically come with a recommendation from the factory regarding which plugs to use. With Japanese cars for instance, the recommended spark plug manufacturer is almost always NGK. Do not ignore the recommendation, and when buying a used car make sure to check out the plugs as they could be extremely worn. Don't expect a whole lot more power, though. It's important to remember that extra power isn't the point of this "mod." It's more about giving your engine what it needs to perform properly.
Tires can make or break the performance of a car. For example: If you have a 1995 Mustang GT with 215 wide tires you'll get 24 mpg and do mad burnouts even with the highly fickle LSD, but bigger tires mean more traction. More traction means more speed, and that means less mpg. But who cares as long as you get to where you're going faster, right? These can be expensive, especially a set of 245s for your '95 Mustang, but resources are available for the frugal spender. A set of tires on Craigslist can be found for a couple hundred dollars or less.
Here's a modification that will cost you exactly zero dollars and about 30 seconds of your life. It somewhat coincides with the cold air intake, but this is more for a high school student with a Nissan 240SX and no job. Which, by the way, is a bad combination. Some cars have at the fender-end of an air intake tube what some mechanics refer to as a "snorkel." In environmental terms it's a kind of funnel that sits at the end of an air intake. For the enthusiast, it's a lump of useless rubber or plastic that takes up space and suffocates airflow. Take it out and unload a ton of horsepower, and it'll make your car sound way cooler. Side note: "a ton" in this context means not very much. But it will sound cool.