I, too, am on a very tight budget. All of my tanks have relatively low light (between 1-2 wpg), and I've done well - in addition to some listed already - with Hornwort, Cabomba Caroliniana, Anacharis (Elodea), and Amazon Sword. Also, the bulbs commonly available at places like Walmart typically do fine. Surprisingly, Java Moss has never done well in my tanks. One of my favs is Elodea, but what happens is that the tiny leaves get further apart on the stem when there is less light.
Another thing I've noticed with most plants I have (especially my Java Ferns) is sometimes the current growth will gradually be replaced by growth that has adapted to poorer light conditions.
One rule to keep in mind is that almost any plant that has a red color to it requires tons of light.
Hope that helps a bit. I'm no expert by any means, but that's my experience.