You can get a rough estimate with simple calculations, but a little more information is needed. How big the subject and how much detail do you want to be able to make out. 200 yds is a long way to make out any detail. Do you want to read license plates, see if he has a mustache or just see the pattern of the plumage. 200 yards will seem like 50 yards with a 200 mm lens, 200 yards will feel like 35 yards with a 300mm lens and 35 yards is quite close for human details. Having figured that out you can pay anywhere from $300 to $3000, so name your budget and go from there. Then it's up t you to learn how to use a long telephoto lens after that. It's not as easy as you might think to get good shots, especially something that may be a one shot opportunity or in difficult circumstances. i'm using the human form as a we are all familiar with thatsize and shape, I could say stock cars but that a different technique all together.