Vahe,
There are a few approaches to answer your question. You asked for the cheapest, which is different than the best bang for the buck.
The best bang for the buck approach is to implement a turbo or supercharger kit, like those from Greddy and others. If you know what you are doing, you could even create your own kit, though you run a zero percent chance of it being smog legal.
The cheapest way to make your car go faster is to upgrade it component by component. This is the route most choose, because it requires investments over time.
My first recommendation is all of the obvious stuff: Reduce the weight in the car by removing unnecessary items and stripping the interior. Find some used rims that are light too. If you search for used parts online, you should be able to find some 15x7s that are around 10-11 lbs. Its a cheap upgrade.
My second recommendation is to get a Kpro. Yeah, its $1000, but you arent going to be able to take advantage of engine modifications without it (like all of those cold air intakes, headers, and other engine mods).Be sure you get it tuned, or do the datalogging yourself and tune it.
Third, before you consider doing drastic things with the motor, pick up a header and cold air intake. It will reduce the weight of the intake parts, allow the engine to breath easier, and will lean out the fuel to air mixture, which you will adjust with the Kpro.
Anything beyond this will require some serious modification... A good bang for the buck upgrade at this point would be to change the head to an EP3 A2 head. It breaths a lot better and can improve power quite a bit.