It's easy to find inexpensive aftermarket air filters for your vehicle. Many of eBay sell for around $40 and promise big horsepower and efficiency increases.

How good are they?

First off, many of the filters sold, either standalone or as part of a cold-air intake kit, do NOT come oiled and do not include oil. The kit I bought did not have any instructions at all and I did not know about the oil until about 20,000 miles later, when my car has severe engine wear and burns oil because the filter wasn't filtering the air. And speaking of which, the kit I bought for my 1989 Honda CRX Si did not even fit, the intake pipe wasn't quite the right shape and even if it was, the whole thing was just stuffed in there without being mounted to the car in any way. I ended up going to Home Depot and getting a rubber plumbing elbow and using my stock intake pipe.

So, these supercheap intakes are against my recommendation based on this experience, although maybe some others will fit and work better.

The thing to be even more concerned about is the filtering quality these filters provide. Stock paper filters are better than any aftermarket cotton-oil-reusable type filter, even the name-brand K&N ones. There is no way to be sure of the quality of a cheap knockoff brand, and the air filter is a relatively cheap element I would not be skimping on, or you might have a oil-thirsty engine like mine after a couple of years.

A problem with most reusuable and oiled filters is that they trade filtering efficiency for airflow, and they generally provide unacceptable levels of filtering when they are first installed. Once dirt begins to accumulate, the holes in the filtering media become plugged up and the filtering efficiency increases, but the initial efficiency is approximately 94% for a K&N cotton filter. That may sound good but stock paper filters are usually over 99%.

My recommendation? If you modify your intake, buy a decent filter. It'll set you back around $50 or so just for the filter but its not worth dancing with engine damage. One that I have had good luck with is AEM's DryFlow filter, which cost about $55 after shipping on eBay. It doesn't need to be oiled which alleviates some problems with airflow sensors and it also has better advertised filtering efficiencies (initial efficiency = 98.4%). Be warned that the advertised horsepower gains are usually heavily inflated. On a stock setup, don't expect more than a 5% gain. Much of the perceived performance increase from air intake kits is the loud, throaty noise they produce. That said, for the money spent, upgrading the air induction system is one of the cheapest ways to increase the performance of a vehicle. Just don't skimp on it.