The expensive part about green-screen effects is the screen itself, and the lighting.
I created a video using a green-screen last year. Inside, it wouldn't work at all, even with 7 lights around the subject. Outside, I got better results, but it still took about 6 hours just to get the chroma key just right.
I used a 4 metre-quare cloth for the screen, pulled tight across the side of a building. Even then, the colour is uneven, and simple things like reflected sunlight change the tones.
Good luck! If you find out how to do it successfully and cheaply, I'd love to know for my own projects
