You may need to buy everything = $300-500.
Depending on the scene, this number could be much lower than the actual amount of $$$ needed.
There are other elements to consider, and a lot of them depend on the vision of the director. It can range from a little (a simple static green screen shot... hell, sometimes you don't even need the green screen, you can rotoscope if you don't mind pushing more work on to your post prod people) all the way up to a full blown action sequence where you need a built up set, stunt crew and set extensions.
Part of the cost will also depend on your access to people who can do this. You may find yourself needing a compositor, sfx coordinator, animator, gaffer, Foley, sound design, color grader and others all to pull off this one shot.
Ultra low budget + special effects + no knowledge on how to do it is rarely a recipe for success or is also rarely cheap.
A lot of green stuff is relatively easy to do and cheap, though, you still need to know what you're doing. Test, test, test. Lighting, grading, performance, quality of the green screen, even sourcing the background will all play a part.