Yup.. a couple of things here, at 1000 mm even the tiniest movement of the camera is going to cause a huge jump in the video - so the camera is going to have to be rock solid stable which means to have much hope in pulling this off you're probably going to need a tripod that will cost you more than what your hoping to spend on the lens.
The only lenses in the 500 mm range you can get for less than $200 are going to be .. well, let me be honest, they will be garbage. Your looking most likely at a prime lens (no zoom) with very bad optical quality, manual focus only with a starting aperture as Scraig mentioned of f 8.0 - add a 2x teleconverter onto that and you lose two F stops of light right there, so your starting at F10 - which means unless your subject is very well lit your video will most likely turn out to be about he same quality as most bigfoot videos I've seen. The minute your subject changes distances (moves) your video will go out of focus and will have to be refocused, and even with a great tripod underneath it you'll most likely cause a huge jump doing that, then you'll have to reaquire - etc.
Really to do this and have any hope of pulling it off you're probably going to have to add another zero onto the end of your budget, spend at least $1000 or more on a 500 mm lens with some sort of image stablization, another $200-$400 on a teleconverter, and another $250+ on a good tripod. Otherwise, well I hate to tell you this but you'd just be throwing your money away.
