I invested 40 Indian Rupees for my DIY CO2 system (Thats less than a US dollar).
Basically I have a yeast system in a pop bottle, attached to a "long" airstone, you know, the kind that have a curtain at the back. The main problem is getting the CO2 to dissolve in the water. Proper diffusers cost more than a 1000 rupees here !!
Now, the CO2 will not form a "curtain" of bubbles from the airstone, because there is not enough pressure. It will basically take the path of least resistance and you will see around 3 places on the stone from where the bubbles emerge.
I have strategically placed plants at these spots, and the bubbles are blocked by the leaves of the plants, that creates a "ladder" like system. Sure enough, I see fewer and fewer bubbles travelling up.
Trouble is, I don't have any testing kits, as they are very costly too, but my algae has started disappearing, and plants have started pearling, so I'm sure that it's working

(well, to some extent atleast)
If you are still skeptical about this, then you can just go in for easier plants that don't require CO2, the ones that Jen has mentioned.