Depends on the test. They run anywhere from five bucks to fifty or more. You need a good test kit to get accurate and reliable readings though. I'd plan on spending 20 bucks.
lol u think spending 200 on fish stuff is alot... sigh..
u def should look into getting bigger tanks and more filtration for sure... everyone here isn't trying to piss you off.. we are only here to help and its your choice to utilize the information provided... it is your sole responsibility to take good care of your fish.. which in this case means bigger tanks or rehoming..
Test kits cost about 1/2 as much on line as they do in stores. Get one with liquid reagents and test tubes, do nit get one with little sticks that that dipped in the water.
API Freshwater Master test kit is a real popular one.