Cheapest ways to increase horse power on a car?

I have a mazda protoge, five speed. but I wondered what are other ways of rising hp withought put a big whole in wallet. besides adding a cold air intake or exhaust system which the car already has

there aint no substite for cubic inches..old american saying I have heard of here in the UK...

The cheapest things to do are already done. You may get a small increase with underdrive pulleys. But the next step is a ECM chip or a standalone fuel system or something like that but that's in the hundreds of dollars too and it will not get you a big get. The only other big get is going to be a turbo or supercharger set up. Or to get into cams and high comp pistons.

buy a better air filter setup
throttle bodies
change oil weights if you're using 10w 40 go to 5w 30
unless you have frigid cold winters.

You can reprogram the cars computer, there are places
and kits that can do that for you.

intake and exhaust

Anything that decreases weight will be the same as rising horse power. The yield for power train weight reductions yield the most per oz.
So get all the junk out of the buggy, as well as that stereo and loose some weight yourself and do not cruise with buddies.
Shift afterward to stay in the power curve so that you use what you have.

There are little things you may be able to do which in general increase the speed and power of a car.
cold air intake
better clutch plate
full exhaust
forced induction like turbo or supercharger pricing is determined on the make of a car.
ex: honda parts are much cheaper than BMW parts.

Also
headers and other engine parts,
V chips which break the limiter on your cars top speed and speed up you are car to about 15 30 hp.
then there's nos
and a bunch of other parts.

Get an import magazine, see what people do to their cars, hopefully it'll give you some ideas.

Good luck